The UK has "no choice" but to shelve its deal to hand over the Chagos Islands to Mauritius in the face of US opposition, a former top civil servant said.
Former Foreign Office permanent secretary Simon McDonald said the government was forced to put the plan in the "deep freeze" as it struggles with an "openly hostile" US President.
A bill to cede sovereignty of the Indian Ocean archipelago is not expected to feature in the King's Speech, which sets the agenda for the next parliamentary session, after US President Donald Trump called the plan "an act of great stupidity".
It is understood the UK has still not had a formal exchange of notes from Washington - a technical step but a legal necessity for the treaty to be enacted.
Officials have not abandoned the plan entirely but time has run out to pass legislation before parliament is dissolved in the coming weeks, and a government source described the situation as "deeply frustrating".
Asked about the delay, Mr McDonald, the top mandarin at the Foreign Office from 2015 to 2020, defended the UK government's handling of the position.
"The government had no other choice. The UK had two objectives, one was to comply with international law, the other was to reinforce the relationship with the United States," he told BBC Radio 4's Today programme.
"When the President of the United States is openly hostile, the government has to rethink, so this agreement, this treaty will go into the deep freeze for the time being."
Asked whether the government could have made better decisions along the way that might have helped realise its plans, he said ministers had been bound by the International Court of Justice, which advised in 2019 that the archipelago should be handed to Mauritius.
He added: "There is a mood to disregard international law. It is being led by the United States.
"Russia, China have always been a bit ambivalent, but the US being equally ambivalent is noticed by everybody.
"The UK has always defined itself as a country which respects, upholds international law, and I think the Government is correct to keep that traditional policy."
Hangover from colonial era
Mr McDonald said the issue was a hangover from the end of the colonial era which had spanned "more than half a century", adding: "It's going to take even longer to sort out now."
Under the terms of the deal the UK would lease back a British-US military base on the archipelago for 99 years, but the process of ratifying the treaty has stalled after it was publicly lambasted by Mr Trump.
Chagossians were forced to leave the central Indian Ocean territory by 1973 to make way for the military base on the island of Diego Garcia.
The US had agreed to the arrangement, but Mr Trump described it as a "big mistake" as his relationship with British Prime Minister Keir Starmer deteriorated over their opposing views on Greenland and the Iran war.
Conservative leader Kemi Badenoch said the agreement should now find its "rightful place - on the ash heap of history".
"That it took so long is another damning indictment of a prime minister who fought to hand over British sovereign territory and pay £35 billion to use a crucial military base which was already ours," she said.
A government source said: "Diego Garcia is a critical strategic asset for both the UK and the United States and securing its long-term operational future is the sole purpose of this deal.
"We continue to believe the agreement is the best way to protect the long-term future of the base but we have always been clear that we would only proceed with US support.
"Delays within the US system mean we no longer have the parliamentary time to complete the necessary steps in this session, which is deeply frustrating following years of intensive engagement.
"The threats to the future operation of the base are real, including the risk of a successful legal challenge, and we will continue to do everything in our power to protect it.
"We continue to believe that the agreement is the best way of doing so and we will work with the US administration and Mauritius to agree a solution that safeguards Diego Garcia and its ability to operate."
