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.