Dear Sir Robert,
Misuse of statistics by the Prime Minister regarding the Chagos Islands surrender deal
I am writing to ask you to investigate claims made by the Prime Minister about the cost of the deal to surrender sovereignty of the Chagos Islands to Mauritius.
Yesterday, when asked about the cost of the deal, the Prime Minister claimed it would be £3.4 billion, even after accounting for inflation. This figure is inaccurate.[1]
Independent analysis suggests that, once a conservative rate of inflation is accounted for, the true cost of the deal is likely to be in excess £30 billion. That’s a difference of £27 billion – a substantial amount that could mislead the public about the real financial cost.[2]
I understand the government has used the Social Time Preference Rate (STPR) to calculate the figure used by the Prime Minister. The figure is therefore a representation of ‘social time preference’, not a representation of the direct cost to the taxpayer. However, the Prime Minister stated that:[3]
‘That is the net cost, and the reason it is put in those terms is because it is how the Government accounts for it, it is how the OBR counts the cost, and it is the way public sector projects are measured. In other words, what’s the net cost is today? And that is £3.4 billion. Obviously over time, with inflation, then that is the net cost’[4]
The Prime Minister has therefore misrepresented the figure by stating that it is a net cost when in reality it is a figure for the social time preference. Such discounting in the public sector is intended to allow the costs and benefits of different policies with varying time spans to be compared on a common basis. But in this case, it is being used as a statistical sleight of hand to hide the true cost to taxpayers of this surrender deal and appears to be a breach in the Code of Practice on Statistics which states that:
‘Statistics, data and explanatory material should be presented impartially and objectively’[5]
Furthermore, the government has failed to publish the source statistics despite the Code of Practice on Statistics stating:
‘Policy, press or ministerial statements referring to regular or ad hoc official statistics should be issued separately from, and contain a prominent link to, the source statistics’[6]
The government’s failure to publish the statistical methodology therefore appears to be in breach of the Code of Practice on Statistics.
As the UK Statistics Authority, your role is to promote transparency and accuracy in the use of public data. I therefore ask you to investigate whether the Prime Minister’s figure follows the Code of Practice on Statistics to make sure that public confidence in public statistics is upheld.
The public deserves clarity on this matter, especially given the government’s cuts to the Winter Fuel Payment and the tax hikes it has imposed on businesses. A transparent assessment will help to maintain trust in official figures.
I am placing this letter in the public domain.
Yours sincerely,
James Cartlidge MP
Shadow Secretary of State for Defence
Footnotes
[1] In response to a question from David Shephard from the Financial Times yesterday, the Prime Minister said: ‘That is the net cost, and the reason it is put in those terms is because it is how the Government accounts for it, it is how the OBR counts the cost, and it is the way public sector projects are measured. In other words, what’s the net cost is today? And that is £3.4 billion’ (Prime Minister and Defence Secretary Statement on Chagos Islands, 22 May 2025, archived).
[2] The Daily Telegraph, 22 May 2025, archived.
[3] GOV.UK, UK/Mauritius, 22 May 2025, link.
[4] Prime Minister and Defence Secretary Statement on Chagos Islands, 22 May 2025, archived.
[5] ONS, Code of Practice for Statistics, 5 May 2022, link.
[6] ONS, Code of Practice for Statistics, 5 May 2022, link.
Related links
Letter from Sir Robert Chote to James Cartlidge MP – Chagos Islands deal