Passengers using Heathrow are facing lengthy delays because Border Force staff are “stressed” and “cannot cope” with the number of people arriving at the hub airport.
A Border Force whistleblower told the BBC there are large amounts of staff on long-term sick due to the demanding nature of the job, with the airport forced to draft in extra staff from ports to help ease delays.
Recently published figures show immigration target times have been missed for the past two months.
"We are being crucified. There is a high percentage of long-term sick due to stress,” the whistleblower said.
"We are being pushed completely and we cannot cope."
He said staff were not given sufficient time to examine passports properly.
"I am not able to thumb through, to see what you have being doing, get a little story for myself, it's not possible," he added.
He claimed some Border Force staff had to start their shift at Dover, drive to Heathrow to do three hours' airport work, then drive back.
Keith Vaz, chairman of the Home Affairs committee, said: "During the peak summer months there is always an increase in passenger numbers.
"It is a no-brainer for those who run Border Force to have made contingency plans to increase staffing levels for this period.
"Rather than deploying officers from policing border security issues additional staff should have been drafted in to deal with passport checks at Heathrow."
A Home Office spokesman said: "Keeping this country safe and secure is a top priority, which is why we undertake 100 per cent checks on everyone arriving in the UK.
"While every effort is made to keep delays for passengers to a minimum, when very large amounts of passengers arrive in a short space of time, it can mean a longer wait while these essential border security checks are conducted.”