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.
It said: "We are aware of an issue impacting the e-gates, which are staffed and operated by Border Force. We apologise for the impact this is having on your journey.
"Our teams are working closely with Border Force to resolve this as quickly as possible."
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The gates allowed 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.