Passengers have faced long queues at Heathrow after security staff were told to self-isolate.
Images posted online show lines of travellers inside Terminal 5, with some describing it as “total chaos”.
One person caught up in the disruption said on Twitter: “Never seen Heathrow so busy. Tried to stay at home or at least deploy maximum social distancing ahead of flying to the US today – seems slightly redundant when this is what greets you at the airport.
“Wish I had dressed in a zorb ball today!”
Another said: “Total chaos at security at Heathrow airport T5 this morning. Never seen anything like it.”
The airport apologised for the inconvenience and said more staff had been “activated” to help passengers.
A spokeswoman said: “Earlier today we experienced some passenger congestion in Terminal 5 departures, due to colleagues being instructed to self-isolate by NHS Test and Trace.
“We have activated additional team members to assist passengers with their journeys and the operation has now returned to normal.
“We apologise to our passengers for any inconvenience caused.”
In recent months, many people arriving at Heathrow have experienced long delays due to extra coronavirus restrictions at the border.
The airport recently opened a dedicated terminal for passengers on direct flights from red list countries, following concerns that they were mixing with those from green and amber nations.
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Travellers on direct flights from red list destinations now go through Terminal 3, although a dedicated facility for processing such travellers is eventually planned for Terminal 4.
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Earlier, the airport said passenger numbers are still nearly 90% down on pre-pandemic 2019 numbers as its chief executive, John Holland-Kaye, called for the government to reopen travel to fully vaccinated people from more countries.
From 19 July, UK residents who have received two jabs will not be required to self-isolate when returning from amber list countries.
Mr Holland-Kaye said: “While it’s fantastic news that some double-vaccinated passengers will no longer need to quarantine from amber countries, ministers need to extend this policy to US and EU nationals if they want to kickstart the economic recovery.
“These changes will be critical for exporters who are losing out to EU rivals and families who have been separated from loved ones.
“We have all the tools to safely restart international travel, and now is the time for Global Britain to take off.”