Downing Street has been forced into a humiliating statement insisting the sovereignty of the Falkland Islands is non-negotiable after Keir Starmer refused to rule out a deal with Argentina.

No 10 insisted British control over the South Atlantic islands was not up for negotiation.

The astonishing move came days after Sir Keir refused to rule out ending the UK’s sovereignty over the Falklands amid an ongoing backlash over his Chagos Islands deal.

Asked to guarantee that no other British overseas territory would be signed away by the Government, the Prime Minister avoided the question, telling reporters that securing the use of the military base on Diego Garcia, a part of the Chagos Islands, was his focus.

He said: “The single-most important thing was ensuring that we had a secure base, the joint US-UK base; hugely important to the US, hugely important to us.”

The Prime Minister’s evasive answer raised fears about not only the Falkland Islands, which are wanted by Argentina, respectively, but Gibraltar and a series of other dependencies.

He announced on Thursday his decision to authorise the handover of the strategically important Chagos Islands to Mauritius.

Sir Keir’s evasiveness prompted Falklands hero Simon Weston to demand that the PM must not surrender the islands.

The ex-Welsh guardsman, seriously burnt in the 1982 war with Argentina, told The Sun on Sunday: “It’s essential to be clear on all British territories they’re not under discussion.

“If the PM comes out and states it, it puts it to bed. It’s called leadership. It’s got to be the easiest display of leadership to say, ‘No, we’re not having it’.”

The Falkland islanders voted by 99 per cent to stay British.

Simon, 63, also said it was a “misstep” to hand over the Indian Ocean archipelago to Mauritius.

Following mounting pressure Downing Street today insisted that the deal to give up sovereignty over the British Indian Ocean Territory (BIOT) was due to the “unsustainable” legal position and had no impact on other disputed territories.

Asked why the BIOT should not be seen as a precedent for other sovereignty disputes such as the Falklands and Gibraltar, the Prime Minister’s official spokesman said: “It’s a unique situation based on its unique history and circumstances, and has no bearing on other overseas territories.”

The spokesman added: “British sovereignty of the Falkland Islands or Gibraltar is not up for negotiation.”

The controversial Chagos agreement will cost the taxpayer hundreds of millions of pounds and will see the UK paying Mauritius until as late as 2164 – much longer than the 99 years that have been made public.

The Indian Ocean Islands have been British since 1814, but their sovereignty will be handed to Mauritius in the deal that the Government claimed would safeguard global security by ending a long-running dispute.

The PM’s spokesman would not be drawn on the cost to the UK taxpayer of the deal which will see Mauritius being given sovereignty over the islands, with a 99-year agreement to secure the strategically important UK-US military base on Diego Garcia.

He said: “The Government inherited a situation where the long-term secure operation of the military base at Diego Garcia was under threat with contested sovereignty and legal challenges, including through various international courts and tribunals.

“You will be aware that the previous government initiated sovereignty negotiations in 2022 and conducted a number of rounds of negotiations. This Government picked up those negotiations and has reached an agreement, which means that for the first time in over 50 years, the base will be undisputed, legally secure, with full Mauritian backing.”

Foreign Secretary David Lammy will face questions from MPs angry about the Chagos decision later today.

Shadow foreign secretary Andrew Mitchell has described the agreement as a “bad deal for Britain”.

The International Court of Justice had previously ruled the UK’s administration of the Indian Ocean territory was “unlawful” and must end, a judgement backed by a United Nations vote in 2019.