Nearly two-thirds of “child” refugees who officials questioned about their real age were found to be adults, Home Office documents show.
Figures show that in the year to September 2015, 65 per cent of the child refugees who had their age disputed were found to be over 18.
It comes after Conservative MPs raised questions about the ages of 14 refugees who were brought to the UK this week from the “Jungle” migrant camp in Calais.
The Home Office has no way of independently verifying the age of child refugees being brought to the UK.
Home Office documents show that if a refugee does not have a birth certificate, a Home Office screening officer can certify them as a child based on their “physical appearance” or “demeanour”.
David Davies, the Conservative MP for Monmouth, has called for rigorous independent checks – such as dental investigations or X-rays – to establish the age of refugees coming to the UK.
In the year ending September 2015, 590 asylum applicants had their age disputed and of those, 574 recorded as having an age assessment.
Of the 574 refugees checked, 371 were found to be adults.
Since 2006 there have been 11,121 age disputes. Of those, 4,828 were found to have been adults.
Home Office documents say that refugees “should be treated as an adult if their physical appearance/demeanour very strongly suggests that they are significantly over 18 years of age”.
However, they make clear that “all other applicants should be afforded the benefit of the doubt and treated as children”.
Mr Davies said: “It’s pretty obvious that migrants who are willing to risk jumping on moving lorries are not going to shy away from lying about their age.
“We have to verify the ages independently with a test. There are two tests - one that involves bone X-rays and one that involves a dental check.
“Currently Government advice is that they need to give these people the benefit of the doubt and that needs to change.
“When I went to Calais there were caravans with notices on how to get coaching on what to say if you need to get into Britain. So I am very concerned about this.
“I’m all for helping innocent children but it is completely ridiculous if we are just going to allow people in based on what they tell us.”
The Home Office on Tuesday defended its age assessment process, stating: “We work closely with the French Authorities to ensure that the cases applying to come to the UK qualify.
“Initial interviews are conducted to gather information on identity, medical conditions and age among other criteria.”
The Jungle refugee camp could be closed imminently after a French court rejected an appeal from aid groups to delay the clearance.
French authorities are expected to empty the migrant camp in Calais in the coming weeks and dismantle it by the start of winter.
A Lille court has rejected a request from aid groups to postpone the closure, arguing that authorities are not ready to relocate its residents.
Charity groups warned that many of the migrants do not want to stay in France and may set up camp elsewhere to continue trying to cross the English Channel to Britain.
The French interior and housing ministers welcomed the court’s ruling and said the camp should be dismantled before winter sets in.
Hundreds more children are expected to arrive this week after a team of British officials were sent to Calais to help French authorities speed up the transfer of minors ahead of the dismantling of the Jungle.
They are understood to include Afghans, Syrians and stateless Bidun children who originated from Kuwait.
Government sources said that officials are “certain” that every refugee who has come to Britain so far this week is under 18 years of age.
A Home Office spokesman said: “We work closely with the French authorities and their partner agencies to ensure all those who come to the UK from the camps in Calais are eligible under the Dublin regulations.
“All individuals are referred to the UK authorities by the France terre d’asile (FTDA) and are then interviewed by French and UK officials. Where credible and clear documentary evidence of age is not available, criteria including physical appearance and demeanour are used as part of the interview process to assess age.”