The Conservative government of Prime Minister Rishi Sunak has made combating irregular migration one of its priorities and has promised to increase migrant expulsions.
Last week, parliament voted in favor of a controversial law allowing for illegal migrants to be sent to Rwanda.
"Luton (airport) was simply unable to cope with the demands placed on it and we were particularly concerned to find that children were placed in crowded holding rooms with unrelated adults," wrote the Chief Inspector of Prisons Charlie Taylor in a report released on Monday.
Most British airports have detention centers where migrants arriving on aeroplanes or those transferred from other centers are briefly held.
More than 17,400 migrants were placed in those centers between June and November 2023.
Close to 3,000 people were sent during that period to the center at Luton airport, north of London, making it the largest such center in England and Wales.
"The most urgent challenge for the Home Office is to find a solution for the unacceptable conditions at Luton," said Taylor in the report.
Migrants were being held for "far too long" in facilities ill-equipped for long stays, he added.
While migrants are only due to remain for a few hours in such centers, Taylor noted that more than a quarter at Luton stayed for longer than 12 hours.
Close to 600 people, including six children, were kept there for more than 24 hours.
In response, a Home Office spokeswoman said the ministry was working "to ensure detention in holding facilities is for the shortest time possible."
She also insisted that "individuals in detention are held in safe and decent conditions."
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