Government Actuary's Department
The question
To ask the Secretary of State for Foreign, Commonwealth and Development Affairs, pursuant to the Answer of 8 January 2026, to Question 101383, on Government Actuary's Department: Freedom of Information, which department commissioned the Government Actuary's Department to make the calculations to discount the £34.7 billion cash term costs using (a) the GDP deflator and (b) social time discounting methodology; and which public body decided to use the social time methodology.
Answered by Stephen Doughty
As set out in the Explanatory Memorandum to the UK-Mauritius Treaty, the Foreign, Commonwealth & Development Office (FCDO) and Ministry of Defence (MoD) calculated the forecast costs of the deal by discounting the sums due to be paid to Mauritius over the duration of the treaty, using the standard Social Time Preference Rate (STPR) as set-out in the Treasury's Green Book. The average annual cost was calculated by applying the Office for Budgetary Responsibility forecast GDP Deflator to the future payments. The Government Actuary's Department was commissioned by the FCDO to review the calculations, which GAD verified at time of signature. In addition to this, the House of Commons Library verified these figures, and the Office for Statistics Regulation welcomed the Government's approach and said that it is in line with intelligent transparency. The Office for Budget Responsibility also confirmed that the discount rates were correct at the time of publication.
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