Sir Keir Starmer’s “national security” justification for handing over the Chagos islands was proposed by one of his closest friends, who represented Mauritius in a case against the UK.

The Prime Minister has repeatedly said there are national security concerns about the UK’s continued ownership of the Chagos islands, a tiny archipelago in the Indian Ocean that hosts a joint UK-US air base.

He has proposed a deal with Mauritius, which would see the UK give away the islands and lease back the Diego Garcia base at a reported cost of £9 billion over 99 years.

On Thursday it emerged that ministers are concerned about telecommunications on the islands, which it is claimed could be disrupted by “legal uncertainty”.

Sir Keir’s official spokesman said the “electromagnetic spectrum” at Diego Garcia which “enables secure communications in the region” would not be able to continue to operate without a deal.

“The very clear advice that we have is that the future operation of the base without a deal would be at risk,” the spokesman said.

The concerns relate to the International Telecommunication Union (ITU), a UN agency that regulates the use of electromagnetic frequencies.

Asked why the UK could be stopped from using the system, the spokesman said: “Because of the legal judgments in relation to the base.”

It is the first time the Government has given a specific national security reason for the Chagos deal, which is under significant scrutiny by the Conservatives and members of US president Donald Trump’s team.