Officials are only challenging the age of child migrants if they look like they are over 25, The Daily Telegraph has learned as it emerged some of those claiming to be under 18 have already been proven to be adults.

Home Office guidance states that asylum seekers should only be challenged about their age if they are found to be "significantly over 18 years of age".

Staff have been instructed that the policy should only be used with "extreme caution" and that they should only verify their age if their appearance suggests they are "at least 25 years old".

It came as a minister revealed that one in 10 of those are claiming to be children in the Calais camp have been found to be adults.