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Passengers at London’s Heathrow Airport were forced to queue for hours at departure gates after more than 100 staff received a coronavirus alert telling them to self-isolate.
The airport said the backlog was caused by an absence of security staff, who were advised to stay at home by the NHS Test and Trace app.
Queues stretched for hours before the airport brought in more staff to help.
It was suggested the problem was a result of staff failing to turn up for work on the morning after England's Euro 2020 football final defeat to Italy.
Traveller Kathryn Wylie said it was “total chaos” in Terminal 5.
“Never seen anything like it,” she wrote on Twitter.
Fellow passenger Laurence Modiano said he missed his flight because he had to wait in a queue for too long.
"Thanks to this mess I missed the only @British_Airways flight of the day to @AeroportoMilano," he said.
"Now rerouted to Linate but my car is at Malpensa! Arrived at @HeathrowAirport at 0730 and will arrive in Milan late tonight."
David Simanoff estimated there were more than 1,000 people in the queue for security checks.
“The Fast Track line, which I’m eligible for, is closed,” he said.
“This is a Newark-like experience at what’s supposed to be a world-class airport.”
Keaten Stone complained that there was limited social distancing.
“Never seen Heathrow so busy! Tried to stay at home or at least deploy maximum social distancing all week ahead of flying to the US today - seems slightly redundant when this is what greets you at the airport,” he wrote on Twitter.
The airport, which is gearing up for a surge in customers with travel restrictions in England scheduled to end on July 19, said it brought in additional staff to clear the queues.
“Earlier today, we experienced some passenger congestion in Terminal 5 departures, due to colleagues being instructed to self-isolate by NHS Test and Trace," a spokesman said.
"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."
Last week, the government said it was looking at changes to its coronavirus app because of the public's frustration with the need for 10 days of self-isolation if they are alerted to a possible exposure to the virus. Two thirds of adults in the UK have had two doses of vaccine.
Businesses fear many staff and customers will need to self-isolate as the government plans to lift capacity restrictions on hospitality venues and other public events in England next week, despite a surge in the number of Covid infections.
Figures released by Heathrow on Monday showed passenger numbers in June were nearly 90 per cent down on pre-pandemic 2019 levels.
Queues at Heathrow have been a regular occurrence since the traffic light system for travel was introduced.
Lucy Moreton, from the ISU union representing border workers, warned staff would struggle to cope with an increase in passengers after the government announced it would ease restrictions for fully vaccinated travellers from July 19.
“It’s a political decision to check 100 per cent of Covid arrivals and that largely is the problem here," she said.
“There still will be queues associated with EU exit, with staff perhaps not being as available, particularly if self-isolation figures surge in the way people are suggesting they might.
“But while we retain the requirement to check four pieces of documentation for every arriving passenger – the e-gate [staff] cannot check the pre-departure test certificate because they’re not uniform, and they can’t at the moment check any vaccination status, if that becomes relevant – so while that remains the political imperative, these queues are going to happen.”