UKSC. State immunity. Diplomatic missions. Sovereign authority. Appellate duty.

The UKSC has considered the extent of state immunity in employment disputes, adopting a dual-pronged test for determining when administrative embassy staff fall outside sovereign immunity protection whilst placing a procedural obligation on appellate courts to address immunity claims sua sponte.

Background

Mrs Costantine, a dual Lebanese-British national employed at the Saudi Embassy’s Cultural Bureau between 2010 and 2018, undertook administrative functions across various roles. Her duties included data entry as a postroom clerk and basic secretarial tasks for the Head of Cultural Affairs. Despite theoretical access to confidential information, evidence confirmed she never utilised such access nor engaged in analytical or governmental decision-making processes.

Following her dismissal, Mrs Costantine alleged religious discrimination and harassment under the Equality Act 2010. The Embassy invoked immunity under the State Immunity Act 1978 (“SIA”), which was rejected at first instance and on appeal. It should be noted that the Court of Appeal dismissed the Embassy’s appeal solely for non-attendance without addressing the substantive immunity question.

Judgment

Lord Lloyd-Jones identified two distinct issues requiring resolution:

On the procedural question, the UKSC held that the Court of Appeal had failed to discharge its statutory duty under s.1(2) SIA to consider immunity despite the Embassy’s non-appearance. This provision creates an obligation for all courts and tribunals to determine immunity questions irrespective of state participation in proceedings, reflecting customary international law principles: Mighell v Sultan of Johore [1894] 1 QB 149.

On immunity, applying Benkharbouche v Embassy of the Republic of Sudan [2017] UKSC 62, the UKSC confirmed that the ET correctly applied the required proximity analysis. The determinative test comprises twin considerations: (a) whether the employment contract was entered into as an exercise of sovereign authority; and (b) whether the conduct complained of engaged sovereign interests.

The UKSC affirmed that Mrs Costantine’s purely administrative functions—whilst performed within a diplomatic mission—lacked the requisite proximity to governmental functions to attract immunity protection. Decisive factual determinations included her lack of analytical responsibilities, absence of governmental decision-making authority, and purely transactional engagement with confidential information.

The Embassy’s novel argument that discrimination claims necessarily implicate sovereign decision-making was rejected. The UKSC noted the remedial focus on compensation rather than reinstatement and the absence of evidence connecting the dismissal to sovereign prerogatives.

Endnote

This guidance on the interpretation of s.16(1)(a) of the SIA 1978 (as amended), establishes that administrative embassy staff fall outside immunity protection unless their functions are “sufficiently close to the governmental functions of the mission” with proximity rather than mere employment being the determinative factor.

Read the full judgment: The Royal Embassy of Saudi Arabia (Cultural Bureau) v Costantine [2025] UKSC 9