Jeremy Corbyn has written to the prime minister condemning her decision to defy a ruling of the UN’s principal court which concluded that Britain should hand back the Chagos Islands to Mauritius.

In a letter to Theresa May, the Labour leader has accused her of disregarding international law and the right of exiled islanders to return to their homeland. His unusual intervention in a highly sensitive case highlights the dangers of Britain’s diplomatic isolation at the UN.

Earlier this year, the international court of justice (ICJ) at The Hague declared the UK’s continued occupation of the Indian Ocean archipelago was illegal.

The UK has retained possession of the Chagos archipelago since Mauritius gained its independence in 1968. The Foreign Office refers to it as British Indian Ocean Territory or BIOT. About 1,500 native islanders were deported so the largest island, Diego Garcia, could be leased to the US as an airbase in 1971. The islanders have never been allowed to return home.

Timeline Chagos Islands Show Hide

France establishes colony Diego Garcia is made home to a French colony using slave labour on plantations Britain takes control Mauritius and the Chagos Islands are ceded to Britain at the end of the Napoleonic Wars in the Treaty of Paris Becomes part of British Indian Ocean Territory Before independence is granted to Mauritius, the UK separates the Chagos archipelago from Mauritius, creating British Indian Ocean Territory (BIOT). Military deal with US Britain allows the US to use the largest island, Diego Garcia, as a military base in exchange for a discount on the purchase of Polaris missiles. Expulsions begin Forced expulsion of around 1,500 Chagossians begin as access to food supplies is restricted. Most are moved to Mauritius or the Seychelles. Compensation is offered Chagossian refugees in Mauritius were paid compensation, and more offers followed contingent on them signing agreements not to return to their homes. Resettlement to the UK British passports are granted to some Chagossians. Many move from Mauritius to Crawley. A UK government feasibility study into resettlement concludes it would be expensive and difficult. Wikileaks revelations A Marine Protected Area is established around the Chagos Islands. Documents published by Wikileaks show a UK diplomat saying that “establishing a marine park would, in effect, put paid to resettlement claims of the archipelago’s former residents”. Government actions ruled illegal International Court of Justice rules that the agreement to separate the Chagos Islands from Mauritius in 1965 prior to decolonisation was unlawful. UN defeat for the UK The United Nations general assembly overwhelmingly backs a motion condemning Britain’s occupation of the islands. The vote, which left the UK diplomatically isolated, set a six-month deadline for the UK to withdraw from the archipelago and for it to be reunified with neighbouring Mauritius.

In parliament on Tuesday, the Foreign Office minister Alan Duncan told MPs: “We have no doubt about our sovereignty over the Chagos archipelago, which has been under continuous British sovereignty since 1814. Mauritius has never held sovereignty over the archipelago and we do not recognise its claim.

“We were disappointed that this matter was referred to the international court of justice, contrary to the principle that the court should not consider bilateral disputes without the consent of both states concerned.”

The ruling was merely an advisory opinion, Duncan said, and not “a legally binding judgment” – even though the UN general assembly had requested that the issue of decolonisation be referred to the ICJ. “The government respects the ICJ and has considered the content of the opinion carefully,” he added.

But in a letter sent to Downing Street on Wednesday, Corbyn wrote: “I wish to express my concern that the government appears ready to disregard international law and ignore a ruling of the international court and the right of the Chagossians to return to their homes.

“To ignore the advisory opinion will set the UK against the entire continent of Africa and dozens of other countries from all continents which supported Mauritius at the [UN] general assembly and before the court, including Brazil, and many Commonwealth countries.

“[Duncan’s] statement suggests the UK-US defence facility on Diego Garcia helps to keep people in Britain and around the world safe. Yet Mauritius, recognised as a valued friend and trading partner, has publicly accepted the future operation of the base in accordance with international law.

“Labour supports the rule of law both at home and abroad, the ICJ and the United Nations and is committed to respecting the advisory opinion in full, so as to ensure that Chagossians are able to return to their homes. I urge the government to cooperate with the government of Mauritius and do the same.”

Earlier Labour’s Eily Thornberry, the shadow foreign secretary, had accused the government of hypocrisy. “[It] claims to believe in a world order based on rules and rights, yet time and again, it shows by its actions that it has total disregard for both,” she said. “The international court of justice has given a clear and unambiguous opinion in respect of the Chagos Islands, which the UN general assembly will soon endorse, and it is simply not good enough for the Tory government to say that because they disagree with that legal opinion, they are entitled to ignore it.”

Philippe Sands QC, professor of international law at University College, London, who represented Mauritius at the ICJ hearing, said: “It’s a slap in the face of the rule of law, the ICJ and the United Nations. It undermines the UK’s ability to say that it is a state that respects the rule of law.

“There will be consequences. The UK is already struggling internationally and this will leave it floundering and isolated.”

The government is one of the major sponsors of a high-profile conference on the importance of international law being held at the Barbican Centre in London later this summer. One of the lectures is entitled Withdrawal from Multilateralism.

David Snoxell, coordinator of the all-party group on the Chagos Islands, said it was “a sad day when the UK, a founding pillar of the ICJ, the international legal system and rule of law, ignores an advisory opinion of the world court which had been requested by the UN general assembly”.

“Clearly the Foreign Office did not intend to consult parliament before a decision was taken [to reject the ICJ ruling] despite repeated requests from the all-party group to do so. This is riding roughshod over parliament.”