They were held for years in a small fenced-off camp, before being brought to the UK earlier this month in what the government described as a “one-off” move in the interests of their welfare.

A spokesperson for the UK government said it was “carefully considering” Monday’s judgement.

Diego Garcia was never a “suitable long-term location for migrants” and the government “inherited a deeply troubling situation that remained unresolved under the last administration for years”, the spokesperson added.

Lawyer Simon Robinson with UK firm Duncan Lewis, which is representing some of the migrants, said “questions need to be answered about how, in the 21st century, this was able to happen”.

The ruling follows a landmark hearing held in a converted chapel on the island in September. The BBC gained unprecedented access to the island and the migrant camp there to cover the proceedings.

Diego Garcia is part of the Chagos Islands, or British Indian Ocean Territory (Biot), an area described as being “constitutionally distinct” from the UK. It is administered from London by a commissioner based out of the Foreign Office.

In their time on the island, the Tamils, including 16 children, were housed in military tents in the fenced camp, which was guarded at all times by private security company G4S.Tamils have described their time on the island as like living in “hell”.

“It’s like an open prison – we were not allowed to go outside, we were just living in a fence and in a tent,” one woman told the BBC after being brought to the UK with her husband and two children this month.

During a site visit to the camp in September, the court saw rips in some of the tents and rats nesting above military cots that the migrants were given as beds.

There were multiple hunger strikes and numerous incidents of self-harm and suicide attempts in response to the conditions in the camp, after which some people were transferred to Rwanda for medical treatment.

There were also cases and allegations of sexual assault and harassment within the camp by other migrants, including against children.

Margaret Obi, acting judge of the Biot supreme court, said in her ruling on Monday that the camp was a prison “in all but name” and “had been a prison from the outset”.

She found that one former deputy commissioner “appeared to have only a limited appreciation of the fundamental importance of liberty”.

Tom Short, a lawyer with firm Leigh Day, said the judgement was “not only a vindication of our clients’ rights but a triumph for the rule of law in the British Overseas Territories.”

“Such an affront to fundamental rights should never have happened and in due course this travesty of administration must be looked at in full,” he added.

The camp has now closed but two men with criminal convictions and another under investigation remain on Diego Garcia, the BBC understands.

Britain took control of the Chagos Islands from its then colony, Mauritius, in 1965 and went on to evict its population of more than 1,000 people to make way for the base.

The judgement comes after the UK agreed earlier this year to hand over the islands to Mauritius in a historic move.

Under the deal, which has still to be signed, Diego Garcia would continue to operate as a UK-US military base but Mauritius would take responsibility for any future migrant arrivals.

The Sri Lankan Tamils are permitted to remain in the country for six months, with financial support from the Foreign Office.

Their journey to the UK marks the end of years of complex legal battles waged over thousands of miles over their fate, but their long-term future remains uncertain.

Most of the group of around 60 migrants have been living in a makeshift camp on Diego Garcia – the site of a strategic UK-US military base – since October 2021, when they became the first people ever to file asylum claims there.

On Monday, a government spokesperson described the move as a “one-off, due to the exceptional nature of these cases and in the interests of their welfare”.

“This government inherited a deeply troubling situation that remained unresolved under the last administration for years,” the spokesperson said.

Tessa Gregory of UK law firm Leigh Day, which represents some of the migrants, said it was the “only sensible solution to end the humanitarian crisis” on the island.

“This vulnerable group which includes 16 children have spent 38 months detained in the most squalid of conditions on Crown land… we hope our clients will now be able to seek safe haven and begin to rebuild their lives,” she said.

At Diego Garcia and the migrant camp there, the Tamils were housed in groups in military tents, some of which had leaks and rats nesting inside.

During their time on the island, there were multiple hunger strikes and numerous incidents of self-harm and suicide attempts in response to the conditions, after which some people were transferred to Rwanda for medical treatment.

There were also allegations of sexual assaults and harassment within the camp.Migrants have said that it was like living in “hell”.

Those in Rwanda have also been brought to the UK, arriving on a flight on Tuesday morning.

The Tamils’ arrival in the UK comes amid uncertainty over the future of the territory.

The UK announced in October that it was ceding control of the Chagos Islands, of which Diego Garcia is part, to Mauritius. But the new Mauritian prime minister has said he has reservations over the deal, which was struck by his predecessor and has still to be signed, and has asked for an independent review.

The deal is facing opposition from some politicians in the UK and allies of US President-elect Donald Trump.

UK Foreign Secretary David Lammy has played down the criticism, describing it as a “good deal” for both Mauritius and the UK, and saying it addresses US concerns about the future of the Diego Garcia base.

In recent years, the territory has been costing the UK tens of millions of pounds, with the bulk of this categorised under “migrant costs”.

The Foreign Office officials, in July warned that “the costs are increasing and the latest forecast is that these will be £50m per annum” if they were to remain there.

In letters sent to the Tamils on Friday from the Home Office, they were told they were being granted temporary entry clearance to the UK “outside of the Immigration Rules” to allow them to consider their “long-term options”.

It stressed that the offer did not “constitute permanent settlement in the UK or recognition of refugee status by the UK government”, and said the group would not be permitted to work.

The government says the Chagos Islands, known as the British Indian Ocean Territory (Biot), are “constitutionally distinct” from the UK, with the unusual status leading to the long legal dispute.

Most of the Tamils have been awaiting final decisions on claims for international protection – which the United Nations says is akin to refugee status – or appealing against rejections.

In total, eight have been granted international protection, meaning they cannot be returned to Sri Lanka.

Successive governments have previously said that bringing the Tamils to the UK would risk creating a “backdoor migration route”.

But the government said on Monday that arrangements had been made to ensure this did not happen, citing a deal to send future arrivals to St Helena – another UK territory some 5,000 miles away.

“Once a sovereignty agreement with Mauritius is fully in place they would then take responsibility for any future migrants,” the spokesperson said.