An asylum seeker has died on board a barge housing migrants off the south coast of England, according to refugee charity Care4Calais.

The death is reported to have taken place on the Bibby Stockholm, a barge which is moored in Portland, Dorset, and which can house about 500 people.

British Prime Minister Rishi Sunak has made cracking down on illegal migration one of his key priorities, and as part of that the government is trying to cut the cost of accommodating asylum seekers in hotels by using barges and former military sites.

But the policy has been controversial. Human rights campaigners compared the barge to a prison ship and said its use was inhumane.

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Care4Calais said in a statement that the government must take responsibility for “this human tragedy”.

“Our thoughts are with the person who has lost their life, their family and their friends,” it said on Tuesday. “They [the asylum seekers on the barge] are being separated from the rest of society and we have witnessed a serious deterioration of people’s mental health.”

Britain’s interior ministry said it was aware of reports about an incident involving someone on the barge.

“It would be inappropriate to comment further at this time,” a Home Office spokesperson said.

The reported death comes as Sunak faces a test of his leadership with MPs to vote on his divisive plan to send asylum seekers to live in Rwanda.

The British Supreme Court ruled last month that Rwanda was an unsafe place to send those arriving in small boats on England’s southern coast, and that the policy would breach British and international law.

In response, Sunak has agreed a treaty with the east African country and brought forward emergency legislation to override domestic and international human rights law.