Heathrow has apologised to passengers for long queues at Terminal 5 on Monday after security staff failed to turn up to work.
Departing travellers took to social media to complain about the delays, with one person reporting 2.5 hours to get through departures and another saying it took 90 minutes to go through security.
Check-in staff told passengers the delay was caused because 120 security staff did not arrive for work.
Heathrow said there had been ‘some passenger congestion’ because several employees had been instructed to isolate by NHS Test and Trace.
The airport said it has since drafted in more staff.
“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,” the airport said.