The UK could lose its permanent seat on the UN security council unless it resolves the future of the Chagos Islands, the archipelago in the Indian Ocean whose largest island, Diego Garcia, is a US military base.

The shock claim is made by David Snoxell, a former British high commissioner to Mauritius from whom Britain purchased the islands in 1965.

The UK’s claim to the islands – and its subsequent eviction of their inhabitants to make way for the US base – is deeply contentious and has been the subject of escalating legal challenges.

Last February the International Court of Justice (ICJ), the principal judicial body of the United Nations, issued an advisory opinion that found the UK was in unlawful occupation of the islands and demanded that they be returned to Mauritius as quickly as possible.

The UN general assembly endorsed the opinion in May and set a deadline for implementation of 22 November 2019, which the UK ignored.

This is an alarming prospect and deeply humiliating for all that the UK has always stood for David Snoxell, former high commissioner to Mauritius

“It must be in the UK’s interest to be seen to abide by international law, especially now the UK is in search of a new role in the world, following withdrawal from the EU and the uncertainty of our special relationship with the US,” Snoxell said. “To continue in breach of human rights and the rule of law will have implications for the UK’s reputation and permanent seat on the security council, as happened in November 2017 when for the first time the UK judge on the ICJ failed to be re-elected. It would relegate the UK to a minor part on the international stage if we lost our seat on the security council.”

Last month, the prime minister of Mauritius, Pravind Jugnauth, told the BBC that he was considering bringing charges of crimes against humanity against individual British officials at the International Criminal Court as a result of the UK’s failure to act.

“It is a violation of the basic principle of human rights. I fail to understand why Britain, this government, is being so stubborn,” he said.

“If the government fails to negotiate a settlement with Mauritius there is a possibility of former Foreign Office ministers and BIOT (British Indian Ocean Territory) commissioners finding themselves before the International Criminal Court, charged with crimes against humanity,” Snoxell said.

“This is an alarming prospect and deeply humiliating for all that the UK has always stood for. It goes against all our principles from Nuremberg onwards. Under Article 7 of the Rome Statute deportation or forcible transfer of a population is defined as a crime against humanity.”

Snoxell, coordinator of the Chagos Islands all-party parliamentary group, said the UK’s position regarding the Chagos Islands had left it with few allies and jeopardised its place on the security council alongside the other permanent members: China, Russia, the US and France.

The council was established after the second world war but Snoxell warned that a combination of the UK’s diminished standing in the world and its flouting of UN deadlines meant it could see another state – potentially India which covets membership – lobby for a permanent place on the council. France enjoys the support of the EU and the other three permanent members are too powerful to remove, Snoxell suggested.

A spokesman for the Foreign Office said: “It’s ridiculous to draw a parallel between the election of a judge for nine years and our seat on the UN security council, which as the label says is permanent and where we continue to be an active, valued and respected member.

“It’s also important to remember that the defence facilities on the British Indian Ocean Territory help to protect people here in Britain and around the world.”