Migrants trying to get into lorries at Calais in France where many attempt to get to UK.
Over the past three years three out of five asylum seekers whose age was checked after they claimed to be under 18 were found to be adults.
The Home Office statistics revealed 2,336 cases where the claims to be a child were disputed and then checked. Of these, 1,403 turned out to be over 18.
More than seven out of ten unaccompanied Vietnamese child asylum seekers whose ages were investigated - around 100 in all - were actually adults. A similar percentage of child claimants from Iran and Iraq whose cases were checked were also discovered to be adults.
Concerns about exploitation of child migrant rules have been growing for the past two years, since the admission of applicants from Calais refugee camps who appeared to be older than children.
Child migrants who have been found after sneaking into Britain aboard trains, lorries or ships are treated differently from adults.
They are taken into local council care and are then offered places in schools or with foster families.
If a refugee who claims to be under 18 does not have a birth certificate or other travel documents, a Home Office screening officer will decide whether or not they are a child based on their 'physical appearance and demeanour'.
Unless the person appears 'significantly' over 18, they will be 'afforded the benefit of the doubt and treated as children' until they are age-assessed by local council social workers. This is to avoid the risk of a child migrant accidentally being placed in adult accommodation or detention.
But on some occasions it meant adults were treated as children - potentially posing a risk to school pupils, foster families or children in care.
Migrants trying to get into lorries at Calais in desperate attempt to seek asylum elsewhere.
Mr Bolt added: 'They received no training to help them make such judgments. Some local authorities were concerned that the Home Office applied its benefit of the doubt policy too readily.'
Among those concerned was Conservative MP Tim Loughton, a member of the Commons home affairs select Committee, who said: 'This has been a problem for some time. We have been a soft touch in too many cases for asylum seekers who abuse our hospitality by elaborating their credentials.
'It is right we give a safe haven to those who are in danger, but too often we have been too trusting.'
Comment: This happens in the US as well: Some of the 'unaccompanied migrant children' arriving at NY airport appear to be adults