Travellers took to social media early on Monday to report significant queues at Heathrow Terminal 5, posting pictures of large crowds unable to socially distance.
The BBC reported more than 100 staff were affected, although it stressed the absences weren’t among Border Force staff.
Shortly after midday, Heathrow said the congestion had been brought under control and normal service had been resumed.
A Heathrow spokesperson told the broadcaster: "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."
