It come as new figures show target times to clear immigration for some passengers were missed in the last two months.

Additional staff have now been redeployed from ports at the Channel to help their airport colleagues cope with delays.

A spokesperson for Border Force said it made “every effort” to minimise delays, but added: "When very large amounts of passengers arrive in a short space of time, it can mean a longer wait while essential border security checks are conducted."

The Home Office has insisted that it has an “agile, flexible and intelligence-led workforce” to respond to needs.

However a long-serving member of south east Border Force staff told the BBC on condition of anonymity: “We are being crucified. There is a high percentage of long term sick due to stress.

"We are being pushed completely and we cannot cope."

Heathrow monitors immigration queues and said Border Force had a target of processing 95% of non-European Economic Area passengers within 45 minutes.

The number fell in June to 89.4% in terminal two, 91.3% in terminal three, 93.3% in terminal four and 94.1% in terminal five, the BBC said.