Part-Time Status Need Only Be an Effective Cause of Less Favourable Treatment

1. Introduction

This unanimous judgment establishes that regulation 5 of the Part-Time Workers (Prevention of Less Favourable Treatment) Regulations 2000 does not require part-time status to be the sole cause of less favourable treatment. It is enough that part-time status was an effective cause—that is, a meaningful causal factor rather than a merely incidental background circumstance.

The appellant was a private-hire driver employed by Data Cars Limited. He worked an average of 34.8 hours per week and was treated as a part-time worker. All drivers paid the same weekly £148 “circuit fee” for access to the respondent’s booking system, irrespective of hours worked. The fixed fee therefore represented a higher hourly cost, and reduced take-home pay proportionately more, for the appellant than for a comparable full-time driver.

The central issue was whether this disadvantage was actionable only if part-time status was the exclusive reason for it, or whether regulation 5 applied whenever part-time status materially contributed to the disadvantage. Lady Simler delivered the judgment, with which Lord Lloyd-Jones, Lord Hamblen, Lord Leggatt and Lord Richards agreed.

2. Legislative Framework

The European framework

The 2000 Regulations were enacted following Directive 97/81/EC and the Framework Agreement on part-time work. Clause 4.1 of the Framework Agreement states that part-time workers must not be treated less favourably than comparable full-time workers “solely because they work part time”, unless the treatment is objectively justified.

Clause 6.1, however, expressly allows member states to introduce provisions more favourable than the minimum protection contained in the Framework Agreement. The Supreme Court regarded this as important: domestic legislation was permitted to confer broader protection than EU law required.

The 2000 Regulations

Regulation 5 provides that a part-time worker has the right not to be treated less favourably than a comparable full-time worker:

• as regards contractual terms; or
• by being subjected to another detriment.

The right applies where the treatment is “on the ground that” the worker is a part-time worker and is not objectively justified. Significantly, regulation 5 does not use the words “solely because”.

The Regulations generally require an actual full-time comparator. They also require the pro rata temporis principle to be applied unless inappropriate: benefits and burdens should ordinarily reflect the proportion of hours worked. Although regulation 5 concerns direct discrimination, it unusually permits an objective-justification defence.

3. Decisions Below

Employment Tribunal

The tribunal initially held that there was no less favourable treatment because every driver paid the same fee. Alternatively, applying the “sole ground” approach in McMenemy, it held that any disadvantage was not solely attributable to part-time status. The fee was said to be the respondent’s method of generating revenue.

Employment Appeal Tribunal

The EAT correctly held that formally identical treatment may be less favourable when the pro rata principle is applied. A fixed fee placed a greater hourly burden on the appellant. The EAT considered the sole-cause test legally wrong but regarded itself as bound by Scottish authority and dismissed the appeal.

Court of Appeal

The majority considered that part-time status needed only to be a substantial or effective cause. Nevertheless, it treated the Inner House’s decision as effectively binding and dismissed the appeal. Elisabeth Laing LJ dissented on the legal issue, concluding that regulation 5 required part-time status to be the sole ground.

4. Summary of the Supreme Court’s Decision

The Supreme Court allowed the appeal and held that:

1. The words “on the ground that” had an established meaning in domestic discrimination law when the 2000 Regulations were enacted.
2. That formulation requires the protected status to be an effective or substantial cause, but not the sole or principal cause, of the treatment.
3. The word “only” in regulation 5(2) means that both statutory conditions must be satisfied: the treatment must be on the ground of part-time status and must not be objectively justified. It does not impose exclusive causation.
4. The singular word “ground” identifies the protected ground—part-time status—and does not mean that no other causal factor may exist.
5. Even if clause 4.1 of the Framework Agreement imposed a narrower sole-cause test, the United Kingdom was entitled to confer broader protection.
6. The appellant’s lower hours were an effective cause of at least part of the disadvantage created by the fixed fee.

The respondent had not sought to establish objective justification before the tribunal. The judgment leaves any detailed assessment of compensation to be dealt with on a just and equitable basis.

5. Legal Reasoning

Established meaning of “on the ground that”

By 2000, discrimination law had firmly established that treatment may be caused by more than one factor. Liability arises where the protected ground significantly influenced the outcome or was an effective cause. Motive, intention and conscious hostility are unnecessary.

The Supreme Court inferred that the decision to use this familiar domestic formulation, rather than the Framework Agreement’s words “solely because”, was legally significant. There was no basis for rewriting regulation 5 to insert a word deliberately omitted.

No mandatory conforming construction

A domestic court should ordinarily interpret implementing legislation consistently with a directive. That principle prevents domestic law from falling below the required minimum. It does not require domestic law to be restricted where the directive permits more favourable protection.

Section 19 of the Employment Relations Act 1999 gave the Secretary of State a broad, free-standing power to protect part-time workers. It was not confined to reproducing the Directive. Its breadth also enabled pay protection to be included, even though EU competence concerning framework agreements did not extend to pay.

The grammatical argument rejected

The respondent relied on the phrase “applies only if” and the singular noun “ground”. The Court rejected both arguments:

• “Only” governs the two cumulative statutory requirements; grammatically, it does not modify the causal test in regulation 5(2)(a).
• “Ground” is singular because the Regulation identifies one protected ground: part-time status.
• The different use of “reason”, “reasons” and “principal reason” elsewhere in the Regulations addresses different legal questions and does not alter regulation 5.

Application to the fixed fee

Equal nominal treatment did not produce equal treatment proportionately. Because the fixed fee did not vary with hours, the appellant paid more per working hour and retained a lower proportion of earnings than a comparable full-time driver.

A worker putting in 43.17 hours—the identified full-time threshold—would have been less disadvantaged than the appellant. That part of the difference was attributable to part-time status. The comparator’s exceptionally high hours, above the full-time threshold, might affect compensation, but did not defeat liability.

Possible business explanations, such as revenue generation or software limitations, were better treated as matters of objective justification. They were not established as independent factual reasons for the treatment and, in any event, the respondent had declined to pursue an objective-justification defence.

6. Precedents Cited

General discrimination causation

Nagarajan v London Regional Transport [2000] 1 AC 501 supplied the central causation principle. A decision may have several causes; discrimination is established where the protected ground significantly influenced the outcome. It need not be the sole or main reason.

O'Neill v Governors of St Thomas More Roman Catholic Voluntary Aided Upper School [1997] ICR 33 similarly held that the protected factor need only be an effective cause. The test is objective and does not turn upon the decision-maker’s subjective motive.

R (E) v Governing Body of JFS [2009] UKSC 15; [2010] 2 AC 728 demonstrated that treatment is discriminatory where the basis of the decision is inherently tied to the protected characteristic, even without conscious prejudice.

R v Birmingham City Council Ex p Equal Opportunities Commission (No 1) [1989] AC 1155 and James v Eastleigh Borough Council [1990] 2 AC 751 confirmed that motive and purpose are irrelevant where the criterion itself necessarily produces differential treatment on a protected ground.

Aspinall v MSI Mech Forge Ltd EAT/891/01 was relied upon for a narrower approach to whistleblowing detriment, but it did not clearly impose sole causation and was inconsistent with the later decision in Fecitt v NHS Manchester [2011] EWCA Civ 1190; [2012] ICR 372. Fecitt held that a protected disclosure need only be a material factor in the employer’s decision.

Part-time worker authorities

McMenemy v Capita Business Services Ltd [2007] CSIH 25; [2007] IRLR 400 had adopted a sole-ground test and referred to the employer’s intention. The Supreme Court held that reasoning to be wrong. The result in that case was nevertheless defensible because the loss of public-holiday benefits depended on the days worked, not on part-time status as such.

Gibson v Scottish Ambulance Service EATS/0052/04, 16 December 2004 was the earlier authority upon which McMenemy relied for the sole-ground approach. That interpretation cannot survive the present judgment.

By contrast, Sharma v Manchester City Council [2008] ICR 623 held that part-time status need only be one reason for the treatment. The Supreme Court endorsed its concern that a sole-cause rule would permit employers to escape liability simply by attaching another criterion to the discriminatory treatment.

European authorities

Wippel v Peek & Cloppenburg GmbH & Co KG (Case C-313/02) [2005] 1 CMLR 9 repeated the words “on the sole ground” but did not analyse multiple causation. It turned principally on the absence of a comparable full-time worker and therefore did not resolve the present issue.

MK v Lufthansa CityLine GmbH (Case C-660/20) [2024] IRLR 74 concerned uniform flying-hour thresholds that disadvantaged part-time pilots. It stressed that clause 4.1 expresses the general EU principle of equality and should not be interpreted restrictively, drawing on Zone de secours Hainaut-Centre, C-377/21, EU:C:2022:530. However, competing explanations were considered as possible objective justifications rather than independent causes.

ZT v Ministero dell'Istruzione e del Merito (Case C-268/24) ECLI:EU:C:2025:526 rejected the proposition that discrimination was excluded merely because only some fixed-term workers were disadvantaged. It supported broad protection but did not directly decide the meaning of “solely because”.

The Supreme Court therefore deliberately left open the precise meaning of “solely because” in clause 4.1. Its decision rests on the independent meaning of the domestic Regulations.

Conforming interpretation and statutory construction

United States of America v Nolan [2015] UKSC 63; [2016] AC 463 was decisive. It establishes that conforming interpretation is required to ensure compliance with a directive, but not to prevent domestic legislation from going further where the directive permits enhanced protection.

The authorities discussed in that context included Marleasing SA v La Comercial Internacional de Alimentacion SA (Case C-106/89) [1990] ECR I-4135, Vodafone 2 v Revenue and Customs Comrs [2010] Ch 77, Swift [trading as A Swift Move) v Robertson [2014] 1 WLR 3438, R (Risk Management Partners Ltd) v Brent London Borough Council [2011] 2 AC 34, Teckal Srl v Comune di Viano (Case C-107/98) [1999] ECR I-8121, and Matthews v Kent and Medway Towns Fire Authority [2006] UKHL 8; [2006] 2 All ER 171. Collectively, they show that consistency with EU law does not justify contradicting the text and legislative purpose of a domestic measure that lawfully provides more.

R (O) v Secretary of State for the Home Department [2022] UKSC 3; [2023] AC 255 and R (PACCAR Inc) v Competition Appeal Tribunal [2023] UKSC 28; [2023] 1 WLR 2594 supported the ordinary textual, contextual and purposive approach to statutory interpretation.

Finally, the Court of Appeal’s treatment of the Scottish decision as effectively binding had followed R (Jwanczuk) v Secretary of State for Work and Pensions [2023] EWCA Civ 1156; [2024] KB 275. That approach has since been disapproved by the Supreme Court. The present judgment now supplies an authoritative UK-wide answer.

7. Complex Concepts Simplified

Effective cause

A factor that genuinely and materially contributed to the treatment. It need not be the only or dominant reason, but it must be more than incidental.

Objective justification

A defence under which the employer must establish objective grounds sufficient to justify the otherwise less favourable treatment. A business explanation does not automatically amount to justification.

Pro rata principle

Pay, benefits and comparable burdens should normally reflect the proportion of hours worked. Charging everyone the same fixed amount may therefore disadvantage part-time workers.

Actual comparator

In most claims under the Regulations, the claimant must identify a real full-time worker employed on the same type of contract and performing the same or broadly similar work.

Conforming interpretation

The principle that domestic legislation implementing a directive should, so far as possible, be interpreted consistently with it. The principle ensures the required minimum; it does not remove additional domestic rights.

8. Impact

The decision materially strengthens protection for part-time workers throughout the United Kingdom. Employers cannot defeat a claim merely by identifying an additional reason for a policy that disadvantages part-time staff. Where part-time status has a meaningful causal role, the employer must address objective justification.

The ruling is particularly relevant to fixed fees, uniform performance thresholds, access to bonuses, leave, training, pensions and other arrangements whose burden or value depends on hours worked. Apparently equal rules must be examined proportionately rather than only in nominal terms.

The judgment does not convert the Regulations into a general prohibition of indirect discrimination. Claimants ordinarily still require an actual comparator, must prove less favourable treatment, and must establish the necessary causal link. Other causes may remain relevant to objective justification and compensation.

9. Conclusion

Augustine v Data Cars Limited establishes a clear UK-wide rule: regulation 5 is engaged when part-time status is an effective cause of less favourable treatment; exclusivity is unnecessary. The decision aligns part-time worker protection with orthodox discrimination causation while preserving the Regulations’ comparator, pro rata and objective-justification requirements.