Australian travellers have been left stranded at one of the world's busiest airports after an air traffic control failure led to flight cancellations across the United Kingdom.

Up to 1,000 flights were cancelled on Tuesday, according to tracking website FlightRadar24, and delays are expected to continue throughout today as airlines work through a significant backlog.

A fault in the flight data processing system operated by the UK's air traffic control provider, National Air Traffic Services (NATS), caused the disruption. The system processes flight plans and other critical data.

Perth woman Monique Whiting and her partner Frank are sleeping on the floor at London's Heathrow Airport after their flight to Iceland was cancelled. The couple boarded their Icelandair flight shortly before 1pm local time and remained on the aircraft for more than four hours before being told to disembark.

"We knew pretty early on that there was a bit of a delay, and I could see through the window that there was a bunch of planes lining up to go," Ms Whiting said.

She said the airline provided snacks and water, but later that afternoon passengers were told the flight would not be leaving. Ms Whiting said Icelandair told passengers to arrange their own accommodation and offered to reimburse them up to a certain amount, but nearby hotels were full.

"A bunch of other things had already been cancelled, and a lot of the close-by accommodation was obviously already booked out," she said. "You can imagine how busy it is and there's lots and lots of people still at the airport. We're all staying here overnight and not getting any sleep."

Debbie Payne was four days into a European holiday when she arrived at Heathrow for an 8:50pm flight home to Australia via Singapore. After travelling to the airport by car, train and the London Underground, she discovered flights had been cancelled.

"People weren't sure what to do," she said. "You just had to wait around and hope."

Ms Payne said passengers were initially prevented from passing through security, while some travellers were stranded aboard aircraft on the tarmac. "There were people on planes who couldn't get off because there were no ramps," she said. "It was hard to know what was going on. I felt sorry for people who didn't speak English and those with young babies."