The US government has blocked a British court hearing from taking place on a British territory, citing security concerns, the BBC understands.

The supreme court of British Indian Ocean Territory (Biot) was due to hold a hearing this week, attended by the BBC, on whether a group of migrants was being unlawfully detained on the island of Diego Garcia.

The island hosts a secretive UK-US military base and access is heavily restricted.

The US last week withdrew consent for lawyers representing the migrants and the BBC to access the island, and said it would not provide those arriving for the hearing with transport, accommodation or food until its concerns had been addressed, the BBC understands.

Late on Thursday night - hours before flights for the first leg of the journey from the UK were due to depart - the court shared with the BBC an order vacating the hearing.

A hearing is now being held in the UK to determine the next steps.

Britain took control of the Chagos Islands, of which Diego Garcia is part, from its then colony, Mauritius, in 1965. It went on to evict its population of more than 1,000 people to make way for the military base.

Agreements signed in 1966 allowed for an initial 50-year period of US use of the territory, plus a further 20 years. The agreement was then “rolled over” in 2016, and is now set to expire in 2036, according to the Biot website.

Biot is administered out of London but is described as being "constitutionally distinct” from the UK.

Mauritius, which won independence from the UK in 1968, maintains that the islands are its own and the United Nations' highest court has ruled that the UK's administration of the territory is "unlawful" and must end.

Most personnel and resources on Diego Garcia are under the control of the US, including the majority of the accommodation and transport on the island as well as restaurants and shops.

The US military commander can refuse access to areas operated or controlled by the US military for security reasons.

The exact nature of the security concerns being raised by the US are not clear, but it is understood they relate largely to a “site visit” being conducted as part of the hearing, which was set to include the migrant camp and several other parts of the island.

Biot's official website states that access to the islands is only permitted to “those with connections either to the military facility or to the Territory’s Administration”.

Diego Garcia has been described as an important strategic base for the US. Earlier this year, two B-52 bombers were sent there for training exercises.

In recent decades, US planes have been sent from the base to bomb Afghanistan and Iraq.

The UK government has confirmed that rendition flights landed on the territory in 2002 to refuel there, though former CIA director Mike Hayden has denied reports that it has ever been used to house and interrogate terror suspects.

Dozens of Sri Lankan Tamils landed on the island in October 2021, becoming the first people to file asylum claims on Biot. Around 60 people, including at least 16 children, remain there as complex legal battles are fought over their fate.

They are housed in tents in a fenced camp, guarded by private security company G4S.

Some have been flown to Rwanda for medical treatment following suicide attempts, and those with successful claims are waiting for a “safe third country” to be identified to resettle them in.

United Nations representatives visited the camp late last year and reported that conditions there amounted to arbitrary detention.

The Foreign Office has previously told the BBC that the island is not suitable for migrants to live on and that it is “working tirelessly to process the migrants' claims for protection and to find a suitable third country for those whose claims are upheld”.

"At all times, the welfare and safety of migrants on Biot has been our top priority,” it said earlier this year.