Passengers arriving at Heathrow airport reported delays of several hours after the self-service passport gates failed.
A witness on the ground said they are now back open but queuing lasted for at least two hours on Wednesday morning.
Travellers at Terminal 5 shared images of long queues on social media, while others said they were held on planes due to the congestion.
One person reported an expected waiting time of two to four hours for arriving passengers.
Confirming the problem on Twitter, Heathrow said the fault lay with Border Force, which manages the gates.
Just before 10am the airport tweeted: “Today’s e-gates issue has now been resolved and the e-gates at Heathrow, which are staffed and operated by Border Force, are back up and running again.
“Our teams remain on hand to get passengers on their way and are working with Border Force to monitor the situation.”
The gates allow travellers with biometric passports to pass through the border without a manual inspection.
It comes less than two weeks after the e-gates failed at airports across the UK.