The US government has blocked the UK from holding a court hearing in one of its own territories, it emerged on Tuesday.

The hearing was due to take place on the remote island of Diego Garcia in the Indian Ocean to consider the fate of dozens of Tamil asylum seekers stranded there for more than 1,000 days who claim they are being unlawfully detained.

Diego Garcia is part of the British Indian Ocean Territory (Biot), over which the UK asserts control. The Biot supreme court was due to conduct a site visit on Monday and then begin to hear the unlawful detention claim on Tuesday.

The Tamils, who fled persecution in Sri Lanka, arrived in Diego Garcia in October 2021 after a boat they were travelling in hoping to reach Canada got into difficulty.

British navy ships rescued them and brought them to the island. Since then they have endured appalling living conditions in a rat-infested tented camp the size of a football pitch, surrounded by a wire fence. They are desperate to be relocated to a safe third country.

The UK has leased part of the island to the US for a military base. The US authorities refused to grant access to some parts of the island, so the site visit was cancelled hours before the judge and legal teams were due to board a flight.

Biot’s deputy commissioner, Nishi Dholakia, said the US authorities would not allow the judge and legal teams to board US military flights to Diego Garcia and would not provide transport, accommodation or food on the island until its “security and operational concerns are adequately addressed”.

The US said it would be “willing to reconsider” the requests if the visit could be “conducted in a manner” that addressed its concerns.

A remote hearing in London was held on Tuesday to decide next steps. One of the Tamils watching via video link from Diego Garcia collapsed three times.

Chris Buttler KC, representing some of the asylum seekers, said the cancellation had had a “devastating” effect on the asylum seekers, some of whom have made suicide attempts due to despair about their life on the island. “For the last five months our clients have been counting down the days. There’s a real risk that the delay will threaten their lives,” he said.

Tessa Gregory, a partner at Leigh Day solicitors, said: “That the British Indian Ocean Territory supreme court has been prevented from sitting in its own territory on Crown land is an extraordinary affront to the rule of law and we trust that the foreign secretary will now do everything in his power to ensure that the hearing goes ahead as soon as possible.”

The events leading up to the blocking of the case came just before the general election, so it was initially under the remit of David Cameron before being taken on by the new foreign secretary, David Lammy.

Foreign Office sources said that the welfare and safety of migrants on Biot was a top priority.