Nearly 100 children have been wrongly identified and detained as adults under Shabana Mahmood’s “one in, one out” deal with France, the Guardian can disclose.
Home Office staff said that of 304 age-disputed cases arising from the scheme over nine months, 94 (31%) – were ultimately found to be minors. Children are supposed to be excluded from the scheme, while government policy limits immigration detention of unaccompanied children to 24 hours.
The figures have emerged in a letter sent to the Home Office minister Anna Turley from Jane Leech, the interim chair of independent monitoring boards (IMBs), which scrutinise prisons and immigration detention facilities.
In the highly critical letter, Leech also said some people detained under the scheme:
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Had been awaiting deportation for more than six months, despite being supposed be held for up to 28 days only “in exceptional cases”.
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Were being readied for removal to France despite having TB or requiring hospital treatment.
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Faced removal after being separated from family members with whom they had travelled, which she said had a “particularly damaging impact”.
The “one in, one out” initiative, launched last August, allows the UK to send somepeople who arrive by small boat back to France while agreeing to take the same number of people from France if they are judged to have a legitimate claim.
Leech said the deal was “placing highly vulnerable, newly arrived individuals, including children and survivors of abuse, in detention”.
In the letter to Turley, she wrote: “Your officials have informed the IMB that 304 age disputes have taken place between 15 August 2025 and 1 May 2026, of which 94 (31%) were assessed to be children.
“The IMB is therefore concerned that child safeguarding mechanisms are ineffective, and believes more should be done to ensure that those whose age is under dispute are not detained in the first place.”
Leech criticised the length of time it took for officials to complete age assessments. “In two cases observed by the Heathrow Board, it took 40 days or more from arrival at the IRC [immigration removal centre] to age assessment,” she wrote.
Referring to the deal as Operation Hillmore, its official name within the Home Office, Leech said she had “significant concerns” about the vulnerabilities of those detained under the scheme.
She wrote: “Many of those who have recently arrived present as psychologically distressed, and Boards have observed records and heard testimonies showing that many are survivors of torture and abuse.”
There was evidence of poor health screening at Manston among the Hillmore detainees, Leech wrote. “Gatwick IRC recently received one individual from Manston with tuberculosis, while a second was refused admission to Brook House because of the critical state of their health, and referred directly to hospital.”
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She said people were being held for lengthy periods despite the scheme being intended to operate at pace, with one man detained for 27 weeks.
Government figures covering 4 August 2025 to 30 June 2026 show that 1,087 people were returned to France under the deal. During the same period, 27,920 people arrived in the UK on small boats.
Imran Hussain, the executive director of external affairs at the Refugee Council, described the figures as “disturbing” and said they were the latest example of children being wrongly treated as adults by the Home Office.
“Too often refugee children are being treated as adults and put into unsafe situations, at risk of abuse and neglect, and exposed to significant safeguarding risks,” he said.
“We call on the government to prioritise professional assessments led by trained social workers, which has been found to be the most accurate way to determine age and therefore avoid these risks.”
A Home Office spokesperson said: “Robust age assessments are essential to maintaining the integrity of our border and ensuring migrants receive appropriate safeguarding and access to services.
“We will continue to strengthen and modernise this process, including through the rollout of cutting-edge AI technology from next year to support age assessment.”
