Looking out through the wire fence surrounding Napier Barracks, near Folkestone, Ali, an asylum seeker from Iran, said he was optimistic that he could “start from zero” in the UK after leaving behind all his previous belongings when he fled his home country.

But, standing near the brick hut that has been his home for the past month, Ali, not his real name, said his current problem was an inability to sleep. He was living in a cramped dormitory with 12 beds and was scared of catching coronavirus.

“You can’t focus on anything,” Ali said. “You can’t sleep. You can’t do anything in privacy.”

Ali’s story and those of other migrants at Napier Barracks — a jumble of low-rise, 19th-century buildings overlooking the English Channel in Kent — have sharpened concerns over the contentious site run by the Home Office, the UK government department handling immigration.

Conditions at the barracks have become a critical battleground over the lawfulness of the department’s approach to handling the cases of people arriving in the UK, often via clandestine, small-boat crossings of the Channel, and seeking asylum protection.

Attention on the issue is likely to increase over the summer as better weather brings an increase in arrivals of people in small boats, of which there were a record 8,400 in 2020.

Last week the High Court ruled that the government acted unlawfully by housing asylum seekers at Napier Barracks, which came into use in September last year.

The Home Office said it had improved the site and explained that it was using the barracks because, amid the coronavirus pandemic, it lacked other options to house people claiming asylum.

However, accounts of current conditions call those claims into question. Over recent months, the department has steadily been refilling the barracks by moving people there from hotels or basic, low-quality flats where asylum seekers are typically housed.

The facility was emptied earlier this year after 197 residents, out of the then 400, tested positive for coronavirus. According to the latest official figures, it held 267 migrants on June 4.

While residents are now offered twice-weekly Covid-19 lateral flow tests, groups working with the migrants fear that the camp’s repopulation could lead to a new, severe outbreak.

Minnie Rahman, campaigns director for the Joint Council for the Welfare of Immigrants, said that, despite signs of “posturing” from the Home Office, there had been “no signs of improvement” at the barracks.

“We know that residents are still being housed in dorms of 14, directly contravening public health advice,” Rahman said. “We also know that sanitation at the camp remains poor.”

The department brought the barracks into use while working on proposals for a substantial reduction in the rights of asylum seekers under the New Plan for Immigration proposals, published in March.

Deighton Pierce Glynn, the law firm that brought last week’s successful challenge, said it had started one action over a case where the Home Office refused to move a vulnerable asylum seeker who had been housed at the barracks. The firm classes as vulnerable anyone who has survived torture or human trafficking, who they say find the barracks’ prison-like atmosphere especially difficult.

“Despite assurances, the Home Office is still sending vulnerable people to Napier and the changes that they’ve made — many of them are fairly superficial,” the firm said.

As well as the overcrowding, one migrant said he had fallen victim to a recent outbreak of food poisoning which charities said had affected 14 residents.

“I got a very bad stomach, then I stopped eating for four or five days,” said one young man, who said he had fled Syria to avoid being forced into President Bashar al-Assad’s army. Others had suffered worse symptoms, he added.

Ali said he had been moved to the site a month ago from the Crowne Plaza hotel near Heathrow airport, London, which the Home Office continues to use to house other would-be refugees.

Ali and the Syrian man were among four migrants from Africa, Asia and the Middle East around the camp who spoke to the Financial Times, describing overcrowded dormitories and inadequate toilet and washing facilities. Their accounts were backed up by the Jesuit Refugee Service, a charity that has had access to the site.

The Home Office said the barracks had provided asylum seekers with a “safe and secure place to stay” during the pandemic and pointed to the increased testing and reduced capacity of the site that was “strengthening” its response to coronavirus.

“Our accommodation providers and subcontractors have made improvements to the site and continue to do so,” the department said.

Meanwhile, the Home Office is fighting other challenges over its treatment of migrants. Kent county council announced on Monday that it had taken the first step towards initiating legal action over the number of unaccompanied child asylum seekers it was being asked to support.

The council said 242 unaccompanied children had arrived as asylum seekers in Kent between January 1 and June 1 but that only 52 had been transferred to other authorities, leaving the county facing what it says is an unsustainable burden.

The Home Office on Thursday announced changes intended to address some of Kent’s concerns, including extra funding for the National Transfer Scheme for moving unaccompanied children between local authorities. Kent county council did not immediately say whether it was sufficiently satisfied with the changes to withdraw its threatened action.

The department has also been issuing migrants arriving from mainland Europe with letters telling them that consideration of their asylum claims will be delayed while the department considers whether they can be returned to another country. However, the UK currently has no agreements with other countries to facilitate such returns.

While there is no obligation under international law for would-be refugees to seek asylum in the first safe country they reach, the Home Office argues that they should do so. It says it has introduced the checks in case there is a country to which asylum seekers can be sent back.

Yet critics said there was little sign that legal and other challenges would deter the Home Office from pursuing what they regarded as a combative approach.

Maddie Harris, co-founder of the Humans for Rights Network, a group working with asylum seekers, highlighted a recent government document which argued that housing asylum seekers in “more generous” surroundings would undermine confidence in the system.

“I think they’re keen on keeping this place open to create an image that shows others who may well wish to seek safety in the UK that the way they’ll be treated will be unfavourable,” Harris said.