More than two-thirds of refugees arriving in the UK who had their age assessed were in fact over 18, Home Office figures reveal.
Data from the year ending in June reveals that of 933 refugees whose ages were disputed, 68 per cent were deemed to be adults.
It comes as the Home Office rules out dental checks to verify the ages of young refugees from Calais, after a call from Conservative MP David Davies to implement such tests.
The Home Office statistics revealed that between January 2006 and June 2016, 45 per cent of 11,847 applicants whose ages were checked were not children.
The Home Office says when refugees do not have documentation to prove their age, the test comes down to "physical appearance and demeanour".
The disclosure also comes ahead of an expected arrival of a second group of refugee children in London.
The Commons Welsh Affairs Select Committee chairman said mandatory checks would reassure the public the system was not being exploited.
But a British Dental Association (BDA) spokesman said: “It is both inappropriate and unethical to take radiographs of people when there is no health benefit for them.”
Doctors of the World executive director Leigh Daynes said: "It's as unethical as it is inappropriate to expect healthcare workers to conduct tests on patients for immigration enforcement purposes.
"Such is the seriousness of Mr Davies's unethical and divisive remarks that we are asking his party chairman to consider disciplinary action."
A series of mainstream UK publications, along with the Monmouth MP, have raised eyebrows over the ages of 14 teenagers who arrived from Calais on Monday to be repatriated with family.
A Home Office spokesman also said its comment that the child refugees look older "because war has toughened them up", was an unofficial quote given to a Mail Online journalist.
The spokesman, who could not confirm how many child refugees were expected on Wednesday, 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.
“We do not use dental x-rays to confirm the ages of those seeking asylum in the UK. The British Dental Association has described them as inaccurate, inappropriate and unethical.”
Frances Trevena, acting head of policy and programmes for Coram Children’s Legal Centre, said today that checking age is "notoriously difficult" and that there is "no way to accurately assess".
"Cultural differences, physical changes through puberty and the effect of trauma can have a significant impact on physical experience," she said.
"Vulnerable children fleeing persecution, often suffering high levels of trauma, will suffer from lazy assumptions made on the basis of their appearance."
French authorities are expected to empty the Calais “jungle” in the coming weeks and dismantle it by the start of winter.
A Lille court rejected a request from aid groups to delay the operation, claiming the country was not ready for the relocation of up to 10,000 migrants.
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