A "human logjam" has formed at Heathrow as crowds of people struggle to get through customs after e-gates failed, with some being left stuck on their planes due to the numbers of people inside.
The unspecified technical problem was confirmed to have hit other airports, including Edinburgh and Manchester, leaving queues to grow as more planes continued to land.
In a statement to Sky News, Heathrow said the issue impacted Border Force's "eGates", which are operated by the Home Office, and affected a number of ports of entry.
One person complained to the airport on Twitter: "Please get yourselves organised. We have been queuing outside customs in a holding queue for over half an hour, nowhere near the front and customs is full."
"Appalling service. We got through Calgary airport within 10 minutes. You should feel shame," they added.
Others shared pictures of long queues at other air and rail ports, including Manchester, where despite the crowd wearing face masks there was no room to socially distance.
"Shocking queues at Terminal 5 this morning! Why aren't the e-gates open?! Like arriving in a third world country!" another said.
Documentary maker Louis Theroux called the queue at Heathrow a "human logjam" and one passenger told Sky News it took them three hours from wheels down to baggage claim.
One passenger told Sky News that his plane was parked on the taxi way at the airport, with its engines shut down.
The people aboard that flight were told that the plane couldn't approach the gates due to "a total system failure across the UK".
The airport said on Twitter that "queue times are at unacceptable levels" and added it has called on the government "to address the problem as a matter of urgency", and told passengers "we do appreciate your patience".
Brigitte Fink complained: "Passengers arriving had to stand in the transit tunnel at Terminal 5, packed in, with no ventilation because border control had an 160+ minute line."
"Even with the failure of the e-system the management of the situation by Heathrow and Border Control has gone from bad to worse with passengers stuck in the transit tunnels. Heathrow staff began handing out cans of water," Ms Fink added.
In a statement sent to Sky News, a spokesperson for the airport said: "We are aware of a systems failure impacting the e-gates, which are staffed and operated by Border Force.
"This issue is impacting a number of ports of entry and is not an isolated issue at Heathrow," they added.
Joe Pike, Sky News' political correspondent, said his flight was finally allowed to disembark one hour and 40 minutes after landing.
Edinburgh Airport confirmed it was among the airports affected, saying: "The UK Border Force (UKBF) IT issue is nationwide and UKBF teams are working to resolve this.
"All available UKBF staff are in the arrivals hall to carry out necessary checks and allow passengers to pass through the border as quickly as possible. Thank you for your patience."
A spokesperson for the Home Office told Sky News: "This afternoon a technical issue affected eGates at a number of ports. The issue was quickly identified and has now been resolved."
Despite the technical issue being resolved, long queues remain, with a number of passengers complaining about being told to stay in their planes rather than disembark and experience the "hellish" conditions inside the airports.
"We have been working hard to minimise disruption, and apologise to all passengers for the inconvenience caused," the Home Office continued.
They declined to respond to how the department would be seeking to help process the passengers stuck inside their planes, and declined to identify the cause of the incident.
Passengers are still landing at Heathrow and being told security queues are too long to allow people to disembark immediately.
Border Force has over 270 eGates at 15 air and rail ports around the country "to enable quicker travel into the UK", it says.
The outage follows additional criticisms of Border Force from Heathrow Airport after delays earlier this month, that time believed to be caused by low staffing levels.