In response to written questions, May also revealed that more than 10 million people had entered the UK in August while the pilot scheme was operating.

The scheme was aimed at reducing checks on citizens of some EU countries.

The ports and airports at which the authorised pilot took place included Heathrow, Gatwick, Calais, Glasgow, Manchester Airport, Aberdeen and Cardiff.

May’s announcement follows the storm around the relaxation of passport checks that saw UK Border Force chief Brodie Clark resign.

Clark quit after he was accused of relaxing immigration controls beyond the limits that May had authorised.

The Home Secretary said the pilot scheme should only have been aimed at European travellers, but that Clark scaled back checks on non-EU nationals without her approval.

Clark denied acting improperly but resigned saying that his position had been made “untenable”.

He has since announced that he will sue for constructive dismissal.

Speculation is rife that May will have to battle to keep her job in light of the revelations.

The Guardian also reports that thousands of passengers from all over the world who arrived on private jets were allowed into Britain over summer without any passport checks, in accordance with official policy.