Nearly 100 Children Wrongly Detained as Adults Under UK-France 'One In, One Out' Scheme

Nearly 100 children were wrongly identified and detained as adults under the UK-France 'one in, one out' migration deal, according to figures disclosed by the Guardian on 13 August 2026. The data, drawn from a letter by Jane Leech, interim chair of independent monitoring boards (IMBs), reveals that of 304 age-disputed cases arising from the scheme between 15 August 2025 and 1 May 2026, 94 — 31% — were ultimately assessed to be minors Guardian.

It is unlawful to detain unaccompanied child asylum seekers in adult detention centres in the UK, and government policy limits immigration detention of unaccompanied children to 24 hours. Nonetheless, the IMB findings document systemic failures to apply these protections under Operation Hillmore, the Home Office's internal designation for the scheme Guardian.

The 'one in, one out' initiative, launched in August 2025, permits the UK to return certain small-boat arrivals to France while agreeing to accept an equivalent number of individuals from France who are judged to have a legitimate asylum claim. 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 Guardian.

Leech's letter to Home Office minister Anna Turley catalogues a series of additional concerns beyond the age-assessment failures. She said some individuals detained under the scheme had been awaiting deportation for more than six months, despite the framework stipulating that detention beyond 28 days should occur only 'in exceptional cases.' One man was detained for 27 weeks under a scheme intended to operate at pace. Some detainees were being readied for removal to France despite having tuberculosis or requiring hospital treatment; the Heathrow Board observed cases where Gatwick IRC received an individual from Manston with tuberculosis, while a second person was refused admission to Brook House due to critical health and referred directly to hospital. Poor health screening at Manston among Hillmore detainees was specifically identified Guardian.

Leech also reported that some people faced removal after being separated from family members with whom they had travelled, which she said had a 'particularly damaging impact.' She described the deal as 'placing highly vulnerable, newly arrived individuals, including children and survivors of abuse, in detention,' and said many of those recently arrived under the scheme present as psychologically distressed, with boards observing records and hearing testimonies showing many are survivors of torture and abuse Guardian.

In two cases observed by the Heathrow Board, it took 40 days or more from arrival at the immigration removal centre to age assessment — a delay that meant individuals who were eventually determined to be children spent extended periods in adult detention Guardian.

The 94 confirmed minor detentions represent the latest and largest figure in an escalating pattern. A Guardian report on 5 April 2026 documented 76 age-disputed children detained under the scheme. By mid-May 2026, JRS UK reported at least 141 age-disputed young people detained since the scheme's commencement. By 24 June 2026, the Humans for Rights Network (HFRN) had identified 174 age-disputed young people detained, 25 of whom had been removed to France Oxford Border Criminologies Blog. Freedom of information data published on 29 May 2026 additionally revealed that lone children were held at UK-run detention centres in France Guardian.

The age-assessment discrepancy has structural roots. Home Office data indicates that local authorities deem young arrivals to be children more than twice as often as border forces do Guardian. Under Home Office policy, an individual's age can be disputed either by the Home Office, usually at the point of claiming asylum, or by local authority children's services gov.uk. The National Age Assessment Board, established under sections 50 and 51 of the Nationality and Borders Act 2022, was intended to bring greater consistency to this process. The Home Office's pre-2019 age dispute policy gave immigration officials broad discretion in deciding how to treat age disputes gov.uk.

A coalition of more than 100 organisations warned in June 2026 that a UK plan to use AI to assess the age of young asylum seekers could lead to more children ending up in adult detention facilities Guardian. In July 2026, a court ruled that Britain acted unlawfully by removing asylum seekers' right to challenge decisions rejecting their claim to be a victim of trafficking, in a case linked to the UK-France migrant deal Reuters. In April 2026, the UK and France agreed a new three-year cooperation framework on unauthorised migration, including small-boat crossings Commons Library.

Imran Hussain, executive director of external affairs at the Refugee Council, described the IMB figures as 'disturbing' and said they were the latest example of children being wrongly treated as adults by the Home Office Guardian.

The broader context here is one of persistent legal and operational friction at the intersection of detention policy, age-assessment practice, and international cooperation. The IMB findings, arriving alongside a judicial finding of unlawfulness on trafficking-status challenges and mounting civil-society documentation of child detentions, raise questions about whether the scheme's design — which assumes rapid age determination, health screening, and removal — is compatible with statutory protections for minors and the government's own detention rules. The data showing a 31% rate of wrongful adult classification among age-disputed cases suggests that the initial assessment performed at the point of arrival is systematically less protective than subsequent review by local authorities. Whether the Home Office's proposed AI-assisted age assessment will narrow or widen that gap is a question the 100-organisation coalition has already flagged. The scheme's extension under the April 2026 three-year agreement means these operational tensions are now embedded in a framework designed to last through 2029.