Migration Watch, a think tank which uncovered the figures, said Channel smugglers were exploiting legal loopholes to encourage young-looking migrants to destroy their documents and claim to be minors.

Unaccompanied children are more likely to be granted asylum and less likely to be detained or removed if their claim is rejected.

The think tank warned it posed a safeguarding threat to other children who shared accommodation with the “adults” and a terrorist risk. Ahmed Hassan, the Parsons Green terrorist, pretended to be 16 and posed as a “model asylum seeker” before setting off a bomb on a Tube train, injuring 69 victims.

Priti Patel, the Home Secretary, is planning to crack down on the move by ensuring scientific and medical checks are required for adults suspected of posing as children. That has provoked criticism from asylum groups who claim they would be unethical, intrusive and traumatising.

But Migration Watch said the plans still allow a young migrant the benefit of the doubt when they are initially assessed. The proposed rules say that where there is doubt, it must be assumed that the individual is under 18 “unless and until a more comprehensive age assessment is carried out”.

Currently, an individual will initially be treated as an adult if their appearance and demeanour strongly suggest they are aged over 25. If there is a dispute, migrants have formal secondary checks, known as the Merton test, which involve assessments by two trained social workers.

‘Deeply disturbing that the asylum system is so open to abuse’

The Home Office data show there were 1,696 cases where the age of the child migrant was called into question in the year to September 2021. Of those, 1,118 – or 66 per cent – were found to be 18 or older.

This was the highest proportion on record, up from 47 per cent in the previous year. It has only previously been above 1,000 in 2008 and 2009