Heathrow passport queues: Staff brought in from ports
"Stressed" Border Force staff are struggling to cope with queues to check passports at Heathrow Airport, according to a whistleblower.
New figures show target times for some passengers to clear immigration were missed in the last two months.
Extra staff have been redeployed from Channel ports to help airport colleagues deal with delays.
The Home Office said it had an "agile, flexible and intelligence-led workforce" to respond to needs.
A spokesman said Border Force made "every effort" to minimise delays.
But he 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."
A long-serving member of south-east Border Force staff, who spoke to the BBC on condition of anonymity, said: "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."
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.
Kevin Mills of the Public and Commercial Services Union said: "Redeploying staff from anywhere, particularly the south-east where we've seen unprecedented levels of clandestine activity trying to cross the Channel, really isn't a good move."
'Respond to demands'
A Home Office spokesman said: "Security is not compromised when staff are deployed from other ports or airports, it is maintained.
"Border Force has been set up to be agile, flexible and intelligence-led, deploying quickly according to need and responding to a variety of demands."
Heathrow, which monitors immigration queues, 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.
Three of the four terminals also missed the 95% target in May.
An airport spokesman said: "Heathrow has been talking to Border Force about how they can improve immigration waiting times during peak periods.
"There is not a trade-off between strong border security and a good passenger experience - at the UK's global gateway we want them to do their part to deliver both."
Analysis by Home Affairs correspondent Danny Shaw:
There have always been difficulties striking a balance between the need for security, in terms of rigorous border checks, and the desire to keep passenger queues down.
In 2011, three senior officials were suspended over claims Heathrow staff were told to relax certain border checks on people arriving at the airport at exceptionally busy times.
The row led to an overhaul of immigration structures, with the Border Force section separated from the UK Border Agency.
But a year later, in the run-up to the Olympics, there were again long lines of passengers at the airport.
The latest problems may partly be due to a dip in staff numbers.
According to the Home Office, there were 8,153 Border Force staff in March 2015; by April this year that number had fallen by more than 500 to 7,651.
Chris Hobbs, who worked at Heathrow as a special branch police officer, called on the new Home Secretary Amber Rudd to recruit more immigration officers.
"There are not enough of them to do the job properly. They have got poor retention rates and the customs side of Border Force work is suffering, which must be good news for those attempting to import drugs and other contraband," he said.
Tony Smith, a former head of the Border Force, said airlines needed to help.
"What we don't control in government is the aircraft coming in, what times they arrive, how many people are on them. So it does require some input from the airports and the airlines about how they can help."
Last week British Airways apologised to customers for long check-in delays at Heathrow and Gatwick.