Furious passengers are trapped in an hour-long queue at Heathrow Airport due to reported problems with automated passport scanners.
Photos posted on social media show hundreds of travellers stuck in a massive queue at Terminal 3 while trying to pass through border control.
The queue was so long that passengers said they couldn't see where it started and staff told travellers it was the worst they had ever seen it.
One passenger claimed four out of five passport control machines were not operating for passengers arriving on international flights at one of the world's busiest airports.
It comes as up to 145,000 British Airways passengers which has grounded about 1,600 flights.
Chris Munns, from New York, said there were "major issues" at passport control and passengers were "fairly agitated" while stuck in "giant queues".
After spending an hour in the queue, Mr Munns told Mirror Online: "We're queued fairly far back into the hallways. You end up in the queue right after deplaning."
He added: "I made it through customs eventually but no-one said anything. I could see they were having issues with the automated passport scanners.
"Many were off at one point."
Mr Munns often travels to London for work and said passengers "normally can zip through".
A Heathrow customer service rep replied to tweets, writing: "Hi there, we are sorry for the inconvenience this has caused you. We are doing our best to get this sorted. We thank you for your patience."
Another passenger, Liz, who posted a photo of hundreds of people trapped in a queue at Terminal 3, wrote on Twitter : "Heathrow arrival passport control queue. What the hell."
She told Mirror Online that most passengers were "fed up" over the delay and staff "refused to comment other than they've not seen it this bad before".
Travellers vented their frustration on social media as they kicked off their week by waiting an hour or more to get through passport control.
One man tweeted: "What's going on with T3 passport control? Been waiting an hour and still very far up the hallway."
Another wrote: "Huge queue in T3 at 7:30am ahead of immigration control into UK @HeathrowAirport @AOA_UK - a practice run for ‘no deal’?"
And another added: "You have a serious crowd control issues at your arrivals hall. Need to sort this out. This is absurd.
"What is being done? Based on this crowd size, seems like it’s going to take hours to get through."
Terminal 3 is used mostly by airlines that are members of the Oneworld alliance with British Airways.
They include American Airlines, Cathay Pacific, LATAM, Japan Airlines and Qantas.
Passengers faced a similar problem at London City Airport on Monday morning.
A spokesman for London City said: "It’s not a UK-wide issue. At London City Airport this morning we’ve had a technical issue with the boarding card gates, just before security, meaning our staff are carrying out manual checks.
"There are queues but these are being dealt with efficiently and passengers are getting through to departures.
"Our IT engineers are aware and on-site, working hard to resolve with the supplier."
"Where possible we'll fast-track any passengers with imminent flights."
The tech disruption came as on Monday and Tuesday - affecting up to 145,000 passengers - as pilots launched a 48-hour strike in a dispute over pay.
Members of the British Airline Pilots Association (Balpa) are taking their first ever industrial action against the airline, grounding hundreds of flights.
BA has offered a pay rise of 11.5 per cent over three years but Balpa says its members wanted a bigger share of the company's profits.
Britain's flag carrier has spent weeks offering refunds to passengers or the option to re-book to another date of travel or an alternative airline.
The airline operates up to 850 flights a day, with most expected to be cancelled.
Passengers at Terminal 5 - BA's hub - said it was a "ghost town" with only a handful of flights expected all day and retail stores closed.
There could be further disruption later this week if climate change activists to go ahead with a planned protest at Heathrow.
The group, calling itself Heathrow Pause, has threatened to ground all flights and throw travel plans into chaos for thousands of passengers by flying toy drones at head height within the airport's restricted zone.
It has vowed to stage the illegal protest at Britain's busiest airport despite the risk of arrest and a jail sentence.
