London and Washington are trying to persuade allies to at least abstain, so as to prevent support for Mauritius reaching triple figures.

The Mauritian mission to the UN believes it has reached that threshold, winning pledges of backing from more than 100 member states.

Such a lopsided defeat would also serve to underline British isolation in a battle that many UN member states, particularly in Africa, see as a last stand to preserve a relic of empire, and at a time at a time when its European Union allies, dismayed by Brexit, are no longer automatically offering support.

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It would also reflect the diminishing persuasive power of the US, which campaigned vigorously for the UK cause.

Last week it took the unusual step of hosting a reception for the UK, attended by over 60 member states, to allow British and American diplomats to put their arguments directly.

The Mauritian ambassador to the UN, Jagdish Koonjul, predicted that the lobbying effort would fall flat, and that much of Europe would either abstain or vote with Mauritius.

“I expect a number of European countries to stand up for the rule of law and show they value and respect the institutions member states have themselves created,” Koonjul said on Monday.

Britain took possession of the Chagos Archipelago in 1814 and held on to the islands after Mauritian independence in 1968, allegedly through coercive pressure on independence leaders.

Three years earlier the UK had secretly leased one of the islands, Diego Garcia, to the US to use as a military base.

The people of the islands, the Chagossians, were forcibly evicted and have been campaigning for their return for decades.

The ICJ advisory opinion in February stated the decolonisation of Mauritius was not lawfully completed because the islands were split off from the territory three years before independence, and declared “the UK is under an obligation to bring to an end its administration of the Chagos Archipelago as rapidly as possible”.

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The Mauritian-drafted resolution that will be voted on at the general assembly on Wednesday calls for the ICJ opinion to be upheld and for the UK to give up control within six months and cooperate in the resettlement of the Chagossians and other Mauritian nationals.

It calls on UN and its agencies “to recognise that the Chagos Archipelago forms an integral part of the territory of Mauritius”.

The UK has said it will not abide by the ICJ ruling, arguing that bilateral disputes should not be presented to the court without the consent of both parties.

It also states that the base at Diego Garcia serves as an essential hub for global stability, used in operations against terrorism.

Mauritius has offered a long-term lease on the base but the US and UK have rejected the offer, arguing it does not exclude a Mauritian veto on future military operations and the leasing of outer atolls to other powers, in particular China.

“This case represents a potentially dangerous precedent for all UN member states, as it could normalise the practice of litigating bilateral disputes through UN general assembly advisory opinion requests, even when a state directly involved has not consented to the jurisdiction of the ICJ,” the US mission said in a statement, which pointed to the “joint US-UK military base on Diego Garcia in the maintenance of regional and international peace and security”.

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Successive, humiliating votes to refer the UK’s territorial claim to the ICJ and in 2017 to deny Britain a judge on the court also reflect growing resentment of the UK’s permanent position on the Security Council.

Professor Philippe Sands QC, who represents Mauritius at the ICJ in The Hague said: “On the day the foreign secretary announces the appointment of a human rights ambassador, his department leads the charge at the UN general assembly to oppose a resolution affirming the right of self-determination, ending British colonialism and upholding the ICJ and the rule of law. A sad day for an isolated, lawless, colonial Britain.”

Richard Gowan, the UN director for the International Crisis Group, warned against interpreting Wednesday’s vote as a sign of a general collapse in UK diplomatic influence.

“On the whole UK diplomacy at the UN has been quite good on a number of issues,” Gowan said.

“But the colonial legacy resonates so deeply at the general assembly, there is very little even the smartest US or UK diplomacy can do about it.”

Earlier this month the Labour leader, Jeremy Corbyn, wrote to the prime minister, Theresa May, condemning her decision to defy the ruling of the UN’s principal court which concluded that Britain should hand back the Chagos Islands to Mauritius.

His letter accused her of disregarding international law and the right of exiled islanders to return to their homeland.

The unusual intervention, suggesting that a future UK administration could switch policy, further undermines UK efforts to persuade its allies of the justice of its cause.