Images posted on social media show long lines of travellers inside Terminal 5.
Kathryn Wylie, 26, from Glasgow described her experience as “chaos”.
“Both security points in T5 were queued back the full way of the terminal,” she told the PA news agency.
We apologise to our passengers
“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 two thousand” people in Terminal 5 while she was there.
She added: “One lady came round with water thankfully.”
A spokeswoman 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.”
Many passengers arriving at Heathrow in recent months have suffered long delays due to extra coronavirus checks at the border.