Lone children were held at UK-run detention centres in France on nearly 300 occasions last year, according to documents released under the Freedom of Information Act.
Data obtained by the Guardian shows they are part of about 900 instances when unaccompanied minors have been detained at British short-term facilities near Calais and Dunkirk over the last four years.
Refugee charities said the numbers were “shocking” and raised concerns about secrecy surrounding the treatment of children held in UK-run facilities in France.
Inspectors last year described “poor” conditions at the sites, while NGOs have said they are “legal and procedural grey zones”.
The short-term holding facilities – Coquelles freight, Coquelles tourist, Calais tourist and Dunkirk – are designed to hold clandestine travellers and those suspected of having incorrect paperwork for no longer than 24 hours.
Despite being run by the UK government, data on who is held by the UK in France is not published as part of official immigration statistics.
FoI documents show there were 284 cases of minors held at the sites last year – a 10% rise on 2024. In that year, there were 258 child cases across the sites, a huge 197% rise from 87 cases in 2023. In 2022, there were 253 children recorded as being held at the sites.
There were 7,454 detentions of adults and children at the sites in 2025, a drop from 9,736 in 2024. In 2023, there were 8,302.
Inspectors raised safeguarding issues at the facilities last year after the authorities failed to locate referrals for two vulnerable child detainees who were subsequently re-trafficked.
In one instance, a 14-year-old girl, who had been found zipped in a holdall in a car, and a 16-year-old boy with a history of being trafficked and abused, were detained but then handed over to the French border police.
About a month later, the girl was taken clandestinely to the UK and held in a warehouse with five other women before she managed to escape, fearing she would be forced into prostitution. The 16-year-old boy was also found in the UK.
The chief inspector of prisons, Charlie Taylor, said at the time: “It was particularly worrying that Border Force could not locate safeguarding referrals of vulnerable detainees, including two children who were subsequently re-trafficked.”
Responding to the figures, Kama Petruczenko, a senior policy analyst at the Refugee Council, said: “These facilities are UK-run and form part of the UK’s border operations so we should be worried that so many children continue to be detained.
“Refugee children are children first and foremost. They should not be held in unsuitable detention settings or exposed to processes that risk their welfare.
“The government needs to explain how so many unaccompanied children came to be detained in these facilities and act to fulfil its duty to protect them.”
Jonathan Ellis, the project director of the Detention Forum, a network of more than 50 UK NGOs, said: “With the government planning to develop their detention estate in France, this lack of accountability and use of established procedures must be addressed urgently.”
Maddie Harris, the founder and director of the Humans for Rights Network, said: “After months if not years of traumatic and violent journeys, it is shocking that hundreds of unaccompanied children have been detained in British-run detention facilities, exposed to further harm.
“Unaccompanied children should never be detained, raising significant questions about how these children came to be in these facilities, and what steps the UK is taking to protect them.”
The figures have emerged as a new French detention centre being paid for by the UK faces a legal challenge from an NGO.
Flemish-Artois Coastal Environmental Defence Assembly, an environmental group known as Adelfa, has launched a legal challenge in France, arguing that the permit for the half-built detention centre in Dunkirk should be withdrawn because it violates local planning rules.
The new facility, part of a £660m deal signed last month, was supposed to be dedicated to detaining migrants caught attempting to cross the Channel in small boats and was due to be opened by the autumn.
The Home Office has been approached for a comment.