Passengers at Heathrow Airport were faced with "chaotic" queues of up to two and a half hours on Monday after security staff were forced to self-isolate, delaying 15 flights.
Images posted on social media showed long lines of travellers inside Terminal 5, with little room for social distancing.
British Airways delayed around 15 flights by 30 minutes to allow travellers extra time to make it through the airport due to the long waits, it is understood.
Airport staff reportedly told travellers that "120 security staff" had been told to isolate after just one colleague tested positive. Heathrow later confirmed that some staff had been told to self-isolate, causing "passenger congestion" in the terminal.
But the airport was unable to confirm how many staff were isolating, how many had tested positive and whether they were contacted by the NHS Test and Trace team or pinged by the smartphone app.
Heathrow said contingency measures to bring in extra staff were stepped up as soon as the shortages were flagged.
Kathryn Wylie, from Glasgow described her experience at Terminal 5 as "chaos", saying: "Both security points in T5 were queued back the full way of the terminal. People easily waiting an hour to get through, flights delayed after boarding to wait for passengers to get on who were caught up at security."
Ms Wylie said there were "easily over 2,000" people in Terminal 5 while she was there and water was being handed out to passengers.
Kathryn Wylie, from Glasgow described her experience at Terminal 5 as 'chaos'
Credit: John Arlidge
Laurence Modiano, who was due to fly to Malpensa, said check-in staff told him the staff shortages was "due to 120 security staff being told on their NHS apps to self-isolate".
He later received an email from BA apologising for the "inconvenience" and confirming that the delays were "due to security staffing issues at Heathrow".
Keaton Stone, a BBC producer on The Sky at Night, tweeted that he had "tried to stay at home or at least deploy maximum social distancing all week" ahead of his flight on Monday morning. "Seems slightly redundant when this is what greets you at the airport," he said, showing pictures of the packed terminal.
Extra staff were brought in to make up the shortfall, a Heathrow spokesman said, adding that the security waiting times had returned to normal by the afternoon.
A spokesman for the airport 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."
Passengers flying business or first class were also unable to use the fast track lane and the dedicated Gold Wing because its security lane was closed.
Many passengers arriving at Heathrow in recent months have suffered long delays due to extra Covid checks.
British Airways is also facing a "significantly under-resourced period", according to a BA staff union memo. "This has been caused by a combination of factors, increased self-isolation, higher than expected customer loads, and of course an increase to the flying programme," it said.
The airline said it was operating its flights as planned.