The sacking of Lord Mandelson from his post in Washington is not just a domestic embarrassment for Sir Keir Starmer — it lands directly on the contested Chagos sovereignty negotiations, and the consequences are immediate.
Mandelson was no bystander in this affair. US sources say he was central to talking the Trump administration into accepting the so-called “Chagos Surrender Deal” that Keir Starmer was so determined to push through.
Officials in Washington confirm that, until Mandelson took up his post, much of the administration had no idea the UK proposal even existed. From the moment he arrived he threw himself into lobbying for it — an effort one source described as running “night and day” — to win American acquiescence. The reasoning behind the urgency was plain: Starmer feared that an outright rebuff in the Oval Office would lay bare just how weak his policy really was. Alongside Foreign Secretary David Lammy, Mandelson set about working to “lubricate” US officials and blunt opposition in Congress.
Back in London, the episode was treated as a measure of Mandelson's clout in Washington and his closeness to Trump. He boasted privately that it was his own efforts that had secured the vital American backing for the agreement. Now that he has been disgraced and swept from the scene, those assurances look distinctly fragile.
A new ambassador — one without Mandelson's tight personal bond to Starmer — gives the US administration a clean opening to look at the question again. That opening took on real urgency this week when the Great British PAC sent a letter to President Trump, signed by more than 40 senior British figures, among them former Cabinet ministers, defence secretaries and security experts.
The letter pulls no punches. Handing sovereignty over the Chagos Archipelago to Mauritius, it warns, would be “a deliberate act of strategic self-harm that threatens both British and American security.”
The case the signatories set out is plain:
- Diego Garcia, the crucial US base in the Indian Ocean, would come under the sovereignty of a state bound by the Pelindaba Treaty, Africa's nuclear-weapon-free zone agreement. Mauritius is a party to it; the United States is not.
- That would create legal ambiguity over the basing of nuclear-capable forces, handing adversaries such as China and Russia an opening to lean on treaty provisions to hem in American operations.
- Mauritian “assurances” are not worth banking on. Under Chinese influence, or amid agitation at the UN or IAEA, Port Louis could be pushed into inspections, disputes, or open denunciations of US deployments.
- Diego Garcia has never been administered by Mauritius. This is not restitution — it is the voluntary surrender of a British Overseas Territory that anchors Western security.
Their conclusion is stark: were the UK and US barred from storing or moving nuclear weapons through Diego Garcia, the damage to deterrence and power projection would be severe. They urge Trump to resist the surrender and to make certain Diego Garcia is kept clear of any Pelindaba Treaty constraints.
With Mandelson gone, the political arithmetic has changed. The “safeguards” he claimed to have won for Starmer rested entirely on his own lobbying and personal connections — and those guarantees have now evaporated. What is left is a badly flawed proposal that Parliament, Congress and the US administration ought to examine all over again.
The Chagos file should be reopened in Washington. The price of looking away is simply too high.
