The new age rules come after the Home Office successfully overturned a 2019 Appeal Court ruling that its 18-year-old threshold was unlawful because it failed to ensure that children were not mistakenly treated as adults.

However, after the Supreme Court backed the Home Office in August last year, a spokesman said: “The guidance will change that to treating them as adults if two Home Office officials think the person looks significantly over 18. It will help officials identify adults attempting to pose as children.”

Kevin Foster, the immigration minister, said: “Single adults who falsely claim to be children in order to seek asylum go on to access children’s services, putting the welfare of children and young adults in school and care at risk.”

If there is a dispute, migrants currently have to have formal secondary checks, known as the Merton test, which involve assessments by two trained social workers.

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

Ministers are now planning to replace the Merton test with new scientific methods, including X-ray checks, for asylum seekers suspected of lying about their age.

Earlier this month, Ms Patel unveiled a scientific committee to scrutinise ways of analysing migrants who claim to be under 18, saying the deception carried out by some asylum seekers was an “appalling abuse of our system, which we will end”.

It comes after a series of troubling cases of adult asylum seekers ending up in schools alongside teenagers, raising serious safeguarding concerns. The Parsons Green terrorist, Ahmed Hassan, posed as a 16-year-old to enter the UK before setting off a bomb on a Tube train in west London in 2017, injuring 23 people.