A change to the rota of Border Force staff which allegedly forced officials to work 12 hours straight has been been partly blamed for the long queues at Heathrow Airport.
According to an unnamed official, changes to guards' shifts saw them going from 10 hours and 25 minutes of work to 12 hours and 25 minutes.
That in turn led them to exhaustion and caused them to call in sick, with 20% reportedly off sick in August.
Shocking pictures emerged at the start of the month showing hundreds of passengers crammed into narrow hallways while waiting in "horrific" queues to get through passport control.
The system was blasted by angry passengers as "inhumane" as the Mirror previously reported how some were forced to wait five hours with no ventilation or nearby access to toilets.
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The Home Office was at that point forced to admit the situation had become "unacceptable" after a pregnant woman fainted while waiting in line.
Speaking to the MailOnline, the Border Force official, whose identify hasn't been disclosed, slammed the rota changes as a "nightmare".
"They made all officers extend their shifts by two hours. So an already long 10.25 hour shift became 12.25 hours," he said.
"Deputy director at Heathrow Raj Johal introduced these new rosters.
"They are highly unpopular amongst the staff, higher officers, senior officers and including the rostering and resource management department."
The official went on to say that up to 20% of staff were off sick last month while generally it would only be half that amount.
"It's only going to get worse," he said.
The Home Office, however, disputes this as it said in a statement to the Mirror that shifts had gone from 8.12 hours to 10.25 for staff on newer terms and conditions, rather than from 10 to 12.
"It is inaccurate to say changes to Border Force staff rotas caused a spike in sickness in August," the statement reads.
"The changes were only introduced on August 22nd, and were made following extensive consultation with staff and the trade unions, including the Immigration Service Union, to improve our operational efficiency and effectiveness
"Our utmost priority is protecting the safety and health of the public. We will never compromise on border security and continue to fulfil our vital function of keeping the border secure and provide the resources needed to do this.
"Border Force regularly reviews staffing requirements to ensure resources are deployed flexibly as and when required to carry out the vital function of border security."
