An eGate outage at airports across southern Britain has sparked travel chaos - with passengers reporting two-hour queues.
Photographs posted on social media on Wednesday showed lengthy lines at Heathrow, Luton and Stansted.
It is the third time the eGates - which were first implemented in 2008 - have gone down in as many months.
The Home Office, which manages the self-service barriers, apologised to all travellers who had been affected.
But the problem has still drawn the ire of some passengers, who took to social media.
One described the situation at Heathrow as a "complete shower", and added that there did not appear to be staff around to help.
Have you been affected by the issue? Email webnews@mirror.co.uk.
Another, Bloomberg reporter Sid Philip, said: "Another day, another outage at Heathrow.
"2 hours from plane to outside the terminal and I was lucky!
"The bizarre bit was for about ten minutes none of the counters were manned as the agents laughed, stood up and left!"
And musician Edward Grint posted: "Stansted or Standstill? Landed nearly an hour ago.
"Looks like it’s going to be well over 2 hours waiting for someone to check my forms."
Paul Knights - who said he had been stuck at Luton Airport since 9.40am - told MailOnline : "Someone has come up and down the queue, they've said it's a national outage at border control.
"The official came up once or twice in this massive queue. I'd say there's probably another hour to go.
"People are patient here, there's no ranting and raving. It's generally quite calm."
Meanwhile, a Heathrow worker told The Times : "I came onto shift and they were working.
"Then they just failed and all we've been told is there is a technical issue. It's becoming a real issue."
There are more than 270 eGates across 15 air and rail ports in Britain, according to the government website.
But the barriers also failed at airports and ports nationwide on September 24 and October 6 this year - and were closed for weeks when foreign leisure travel resumed on May 17.
There have also reportedly been staffing issues at the UK Border Force after being in close contact with someone who tested positive for Covid-19.
Commenting on Wednesday's issue, a Home Office spokesperson said: "We are aware of a technical issue affecting eGates at a number of ports.
"We are working to resolve the issue as soon as possible and apologise to all passengers for the inconvenience caused."
E-gates are used to process the vast majority of British and European arrivals.
They are also able to process arrivals from the US, Canada, Australia and Japan.
