The Home Office is facing a judicial review over conditions at the Manston migrant centre in Kent, the immigration minister says.
Robert Jenrick told Sky News the legal action had begun after reports of severe overcrowding.
Council leaders from across Kent have told the Home Office they are “at breaking point”, after new facilities keep opening up to process migrants and hotels are booked up to house them.
Hundreds of people are thought to have been moved out of the disused airfield site near Ramsgate following concerns it had become dangerously overcrowded this week.
But migrants held at Manston have begged for help, describing the conditions as a prison.
A young girl threw a bottle containing a letter over the perimeter fence, claiming there were pregnant women and sick detainees at the Kent facility.
Manston, in Thanet, north Kent, is meant to house up to 1,600 people but has been housing about 4,000, according to MPs.
Mr Jenrick told The Take with Sophy Ridge programme: “I believe we have received the initial contact for a judicial review.
“That’s not unusual – this is a highly litigious area of policy but of course, as the minister responsible I want to make sure everything we do is conducted appropriately and within the law.”
The 14 Kent local authority chiefs accused the department of an “unsustainable and unacceptable” response to the crisis, demanding ministers “stop using their county as an easy fix”.
The leaders told the government they were under disproportionate pressure because of their location.
Mr Jenrick said he could not reveal who had brought the judicial review as it was legally sensitive.
The minister, who was appointed to the job last week by Rishi Sunak, said he has been working with home secretary Suella Braverman to reduce the numbers of people and the time they were staying at Manston – which is only meant to be 24 hours but has been much longer in some cases.
He added: “So the week I’ve been in post I’ve tried to work night and day to ensure the Manston site is not just legally compliant but is a humane and compassionate place where we welcome those migrants, treat them appropriately and then they leave quickly to alternative accommodation.
“I’m pleased to say that this evening that’s the path we’re on, the numbers at Manston have fallen very substantially since the weekend when we became aware of the specific issues and got involved so directly.”
Four senior MPs have piled further pressure on the Home Secretary to explain how the government will get to grips with the migrant crisis.
The parliamentary committee chairs have jointly written to Suella Braverman calling for clarity on how the Home Office will cut the number of treacherous small boat crossings and reduce “as a matter of urgency” the backlog in cases currently within the asylum system.
In their letter to Ms Braverman, the committee chairs call for clarity on what policies the Home Office has in place to address the conditions at the centre and ensure it does not face such overcrowding again.
The letter is signed by the chairs of the Home Affairs Committee, Justice Committee, Joint Committee on Human Rights and Women and Equalities Committee, and requests a response by November 16.