More than two-thirds of refugees who had their ages assessed were found to be adults despite claiming to be children, Home Office figures show.
Tory MP David Davies said 14 teenagers who arrived in the UK from Calais on Monday “don’t look like ‘children”.
Data from the year ending in June reveals that 1,060 asylum applicants’ ages were called into question. Of the 933 who were recorded as having an age assessment, 636 (68%) were deemed to be over 18.
From January 2006 to June 2016, 11,847 applicants were assessed for their age, of whom 5,278 (45%) were found to be over 18.
Mr Davies was condemned by dentists after calling for tests on teeth to verify the age of child migrants.
The British Dental Association (BDA) said demands by the Monmouth MP for testing on those heading to Britain from the Jungle refugee camp were “inappropriate and unethical”.
Mr Davies, chairman of the Commons Welsh Affairs Select Committee, said mandatory dental checks would reassure the public the system was not being exploited.
But a BDA spokesman said: “We are vigorously opposed to the use of dental X-rays to determine whether asylum seekers have reached 18.
“It’s not only an inaccurate method for assessing age, but it is both inappropriate and unethical to take radiographs of people when there is no health benefit for them.
“X-rays taken for a clinically justified reason must not be used for another purpose without the patient’s informed consent, without coercion and in full knowledge of how the radiograph will be used and by whom.”
Doctors of the World, which works in Calais, condemned Mr Davies’s stance as “unethical, inappropriate and divisive” and called for disciplinary action to be taken against the MP.
Leigh Daynes, the charity’s executive director, said: “It’s as unethical as it is inappropriate to expect healthcare workers to conduct tests on patients for immigration enforcement purposes.
“Health staff are not border guards; and in any event such tests have been shown to be unreliable.”
She added: “Such is the seriousness of Mr Davies’s unethical and divisive remarks that we are asking his party chairman to consider disciplinary action.”
Mr Davies defended his stance, saying the authorities should not be “naive” about the issue of adults trying to get into the UK.
He said refugees who had been through an ordeal to reach the UK would not be concerned about having their age checked.
“We must not be naive about this. It’s no good Lily Allen turning up with tears in her eyes and all the rest of it – we need to be quite hard-nosed here,” he told the BBC Radio 4 Today programme.
“People are desperate, I understand that, and they will say what they need to say to get in.
“When I was in the camp in Calais there were caravans with notices on saying ‘Come here, we will coach you in what to say to get into the UK’.”
He added: “People in Britain, I think, want to help children but we don’t want to be taken for a free ride either by people who seem to have got to the front of the queue even though they clearly look, in some cases, a lot older than 18.”
Mr Davies also said he did not accept that it was “intrusive” to take an X-ray of a migrant.
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.
