Passengers landing at Heathrow Airport in London faced queues stretching a quarter of a mile at the weekend as officials carried out spot checks on travel documents.
Travellers complained on Twitter that bottlenecks caused lines “stretching the full length of Heathrow Terminal 5” on Sunday.
The delays occurred the day before the UK loosened its travel rules to allow passengers fully vaccinated in the US and EU to avoid quarantine in the UK.
Millions of tourists are expected to take advantage of the changes, with quarantine-free deals expected to be agreed with other countries in coming months.
Passengers criticised the current arrangements at the airport as inadequate, raising questions over whether Border Force officials could handle a large surge in arrivals.
Carmen Ogilvie posted a video of waiting travellers, asking: “Ridiculous queues for immigration at Heathrow Airport. Is this the new normal?
“What's the point of all these safety measures if this is our arrivals?”
Tom Kibasi, who was also caught up in the delays, said the snaking queues were the equivalent to about a quarter of a mile.
When Heathrow Airport informed him that the queues were because of spot checks from customs officers, he replied: “Great job! I’m sure there are so many benefits from spot checks that they surely outweigh the Covid-19 risk of confining thousands of people in the same space for an extended period!”
The airport said that queues had “on occasion been unacceptable” but said the spot checks were required by the UK government.
Authorities recently changed travel rules to require airlines to check passenger documents before boarding, meaning fewer checks are carried out by Border Force workers at the gate. While officers no longer need to inspect every form, spot checks still take place to ensure passengers and carriers are complying.
“Our teams in the terminals are on hand to support where possible and we are working with Border Force to reduce delays as soon as possible,” Heathrow Airport said.
“Waiting times at the border have on occasion been unacceptable and we have called on the UK Government to address the problem as a matter of urgency.”
A report in The Times claimed the Border Force was hit by a wave of absences at the weekend, with about 80 of the force's 300-strong staff off with coronavirus.
It said dozens more were forced to isolate because they had been in close contact with one or more of them.
The Home Office said last month passengers should expect longer queues during the pandemic.
“Our utmost priority is protecting the health of the public and our enhanced borders regime is helping reduce the risk of new variants being transmitted,” a spokeswoman said.
“That means, for the time being, passengers will need to accept an increase in the time taken at each stage of their journey.”
Last month, passengers at Heathrow were forced to queue for hours at departure gates after more than 100 staff received a coronavirus alert telling them to self-isolate.