Conservatives have demanded Sir Keir Starmer apologise for “deeply offensive and ill-judged” comments after China congratulated Mauritius for convincing the UK to hand over the Chagos Islands. Shadow Defence Secretary James Cartlidge has written to Number 10 “to request a formal apology” after Sir Keir said China opposed the deal – and accused the Tories of taking China’s side.

The agreement means the UK will pay at least £120 million-a-year for 99 years for control of the vital Diego Garcia base. as well as handing over £1.125 billion for economic development over a 25-year period. The the true cost is likely to exceed £30 billion in cash terms.

Announcing the deal on May 22, Sir Keir said: “In favour are all of our allies: the US, NATO, Five Eyes, India. Against it: Russia, China, Iran and, surprisingly, the leader of the opposition and Nigel Farage are in that column alongside Russia, China and Iran, rather than the column that has the UK and its allies in it.”

But the Mauritius Deputy Prime Minister has now expressed gratitude to China “whose unwavering support played a critical role” in a press conference. Meanwhile Mauritius newspaper Le Mauricien reports Chinese diplomats praised the deal as an “historical achievement”.

In addition, China submitted a statement to the International Court of Justice in 2018 on the subject of “the Separation of the Chagos Archipelago from Mauritius in 1965” in which it said: “China has fully understood and supported Mauritius’ legitimate quest for decolonization”.

Writing in the South China Morning Post, US academic Nong Hong said: “In its written statement to the International Court of Justice, China reiterated its firm support for the decolonisation process and expressed understanding of Mauritius’ position.”

Mr Cartlidge said in the letter to Sir Keir: “I am writing to request a formal apology following your recent statement that somehow those of us opposed to your Chagos deal are on the side of Russia, China and Iran – a deeply offensive and ill-judged claim.”

He added: “Your suggestion that HM Opposition is somehow siding with China has now been proven to be factually inaccurate as they support your deal – but when we have supported you so strongly on Ukraine, and backed your military action against Iranian-backed Houthis, to be accused effectively of siding with China, Russia and Iran is totally unacceptable.

“I look forward to your apology.”