“Reasonable Basis” Review: The Supreme Court Clarifies SIAC’s Role in National-Security Appeals – Commentary on U3 v Secretary of State for the Home Department [2025] UKSC 19

1. Introduction

The United Kingdom Supreme Court’s judgment in U3 v Secretary of State for the Home Department tackles a critically sensitive area of law: the interface between national-security decision-making and judicial oversight. The decision refines, and in some respects re-balances, principles earlier sketched in Rehman (2001) and more recently in Begum (2021). At its core lies a procedural question: when the Secretary of State deprives a person of citizenship or refuses entry on national-security grounds, what exactly must the Special Immigration Appeals Commission (SIAC) do on appeal? Should SIAC make its own findings “on the balance of probabilities”, or should it review the Secretary of State’s assessment using public-law principles?

U3—a British/Moroccan national who lived in ISIL-controlled Syria—was deprived of citizenship in 2017 and later refused entry clearance to rejoin her children. SIAC and subsequently the Court of Appeal dismissed her appeals. The Supreme Court was invited to overturn those rulings and to expand SIAC’s fact-finding obligations. It declined, crystallising a new “reasonable basis” review model for national-security cases.

2. Summary of the Judgment

The Court (Lord Reed delivering the only opinion, with Lords Hodge, Lloyd-Jones, Sales and Stephens concurring) dismissed U3’s appeal. Key holdings include:

Distinct Functions: Appeals under ss. 2 and 2B of the Special Immigration Appeals Commission Act 1997 are true appeals, yet SIAC’s function varies with the issue. Some issues (e.g., statelessness) require classic fact-finding. National-security risk assessments do not.
Standard of Review: For national-security assessments SIAC asks whether the Secretary of State’s evaluation has a rational evidential basis and is free from public-law error. SIAC does not on that issue substitute its own risk evaluation.
Evidential Scope: Unlike judicial review, SIAC may consider material emerging after the impugned decision, because the Secretary of State keeps the assessment “under review” during the appeal. Post-decision evidence is therefore relevant.
Human-Rights Compatibility: The “reasonable basis” approach satisfies Articles 6 and 8 ECHR as interpreted in IR v UK, Khan v UK, and Ramos Nunes.
Outcome in U3: SIAC had correctly applied public-law standards, found no irrationality, balanced the children’s Article 8 interests, and was entitled to dismiss both appeals. The Court of Appeal’s single immaterial misdirection did not affect the outcome.

3. Detailed Analysis

3.1 Precedents Cited and Their Influence

Secretary of State v Rehman [2003] 1 AC 153
– Established that national-security risk is an evaluative judgment not amenable to strict proof. The House of Lords cautioned appellate restraint and emphasised deference to executive expertise.
R (Begum) v SIAC [2021] UKSC 7
– Confirmed SIAC’s review role and distinguished between “precedent facts” and evaluative assessments. However, ambiguity remained about how far SIAC could/should engage in its own fact-finding.
Pham [2015] 1 WLR 1591; N3 [2025] UKSC 6
– Demonstrated that some SIAC issues (e.g., statelessness) demand full fact-finding.
Article 6 & 8 Strasbourg Authorities
– Cases such as Al-Nashif, Bryan, IR, Khan, and Ramos Nunes collectively show that limited, legality-focused judicial review can satisfy Convention fairness requirements where technical expertise and public-safety discretion are central.

The Supreme Court synthesised these strands, preferring Lord Hoffmann’s Rehman approach over Lord Slynn’s hybrid model and using Begum chiefly for confirmation rather than innovation.

3.2 The Court’s Legal Reasoning

Issue-Sensitive Methodology
The Court drew a crucial distinction:
- When the statutory question is a concrete fact (e.g., “would deprivation cause statelessness?”) —> SIAC must find facts on the balance of probabilities.
- When the question is an evaluative risk assessment entrusted to the Secretary of State —> SIAC reviews legality and rationality.
No Binary Proof in Risk Evaluation
Borrowing Lord Hoffmann’s In re B analogy, the Court explained that proving discrete facts to a 51% threshold is an inappropriate tool for measuring probabilities of future harm. Even a significant possibility of terrorism may reasonably justify precautionary measures.
Weight to Executive Expertise
Both institutional (access to classified intelligence) and constitutional (democratic accountability) reasons compel courts to assign “very considerable weight” to the Secretary of State’s view, unless vitiated by public-law error.
Human-Rights Overlay
The Court maintained that SIAC must independently evaluate Article 8 proportionality, but still employ the Secretary of State’s risk assessment as the relevant security premise, unless irrational.
Error Analysis
SIAC’s single mischaracterisation—limiting certain evidence to the entry-clearance appeal—was harmless because the same evidence was ultimately tested against the updated security assessment.

3.3 Likely Impact of the Judgment

Procedural Clarity: Practitioners now have an unequivocal template: identify the statutory type of issue; if it is a risk-assessment ground, frame challenges in public-law terms (irrationality, irrelevant considerations, misunderstanding of fact) rather than demanding de-novo fact-finding.
Reduced Forensic Ambiguity: The decision rejects the “pivotal/non-pivotal fact” distinction floated by the Court of Appeal, preventing arguments that SIAC must pick out and re-decide every major allegation.
Strategic Shift for Appellants: Future appellants will likely focus on:
- demonstrating that critical items of intelligence are untenable or misinterpreted,
- showing that the Secretary of State failed to revisit her assessment after new evidence emerged, or
- arguing disproportionate impact under Article 8 notwithstanding an established risk.
Institutional Stability: The Court’s human-rights analysis buttresses the existing closed-material / special-advocate regime against procedural fairness attacks, at least where national-security discretion is centrally engaged.
Comparative Influence: Other common-law jurisdictions with similar special-tribunal models (Canada’s Security Certificate regime; Australia’s AAT) may cite this judgment to justify legality-focused review of security assessments.

4. Complex Concepts Simplified

Balance of Probabilities: A civil-law standard requiring a fact to be “more likely than not” (51% probability) before it is treated as legally established.
Evaluative Judgment vs. Precedent Fact: A precedent fact is a concrete event the law demands be proved (e.g., “Was the applicant born abroad?”). An evaluative judgment is a holistic assessment (e.g., “Does the applicant pose a danger?”) requiring discretion and weighting of possibilities.
Public-Law Error: Mistakes that render an administrative decision unlawful; includes irrationality (Wednesbury), irrelevant considerations, procedural unfairness, or misunderstanding an established fact.
Closed Evidence & Special Advocates: Sensitive intelligence shown to SIAC and security-cleared counsel (special advocates) but not to the appellant directly, balancing secrecy with adversarial process.
Reasonable Basis vs. Proof: The Secretary of State’s decision must be reasonable in light of the evidence, not proved “true”. Think of weather forecasts: you act on credible probabilities, not certainties.

5. Conclusion

U3 cements a two-track analytical framework for SIAC: ordinary fact-finding for binary statutory questions, and “reasonable basis” review for national-security risk determinations. The ruling re-affirms executive primacy in safeguarding security while preserving robust, if legally-structured, judicial supervision. By aligning domestic doctrine with Strasbourg jurisprudence, the Supreme Court has both fortified SIAC’s procedural architecture and provided clearer signposts for future litigants operating in the legally charged terrain where individual rights meet collective security.