Heathrow Airport has apologised for disruption after the west London hub was hit by "technical issues".
One passenger said the situation was "utter chaos" after a problem with the airport's IT system saw staff called in to help passengers get to gates on the second day of the half-term weekend.
At about 22:25 GMT on Sunday, Heathrow said the issues had been resolved and "systems are returning to normal".
British Airways, the biggest airline at Heathrow, has cancelled 20 flights.
In a tweet, Heathrow Airport said: "Today's technical issue has now been resolved and Heathrow's systems are returning to normal.
"We apologise for the inconvenience caused.
"Our teams will continue to monitor our systems and be on hand to provide assistance to passengers as we work to resume our regular operations."
Air traffic control was not affected by the technical failures, but the IT issues, which came on a busy day for family travel, have further compounded delays triggered by bad weather across the weekend.
Sam Mills, who was travelling from London to Pittsburgh with British Airways, explained how when he arrived at the airport shortly after lunchtime on Sunday the flight boards were not updating.
"I was continually getting 'Delayed' messages on the board, with no gate information for my flight," he told the BBC. "A BA representative informed me that it should update before my flight, and not to worry."
"But as soon as the gate did pop up [on the board] - it told me the flight had departed, without me on that plane.
"As of right now, I am stranded. There's a line of people about 300ft in both directions at the British Airways service desk. We haven't been told any information from anybody."
Caitlin Gould, who travelled to London from Cornwall on Sunday morning, has been waiting for a flight to Munich with Lufthansa since 16:00 GMT, after her British Airways flight was cancelled.
She said the staff were "really helpful... if you can find them".
"At the gate there is almost no information," she told the BBC, adding that everyone was dependant on white boards to find out where they should be.
"None of the online information matches up with any of the boards. People are walking around with signs trying to find people to take them to the plane."
British Airways said the cancellations were the result of Heathrow's IT issues combined with the existing disruption caused by Storm Dennis.
It added that anyone on a cancelled flight would be entitled to a refund or could be re-booked. Overnight accommodation would be provided if necessary.
In response to a customer on Twitter, the airline wrote: "We're aware Heathrow Airport is currently experiencing a technical issue that is impacting some of their IT systems across the airport, affecting a number of airlines.
"We are working with them to resolve the issue as a priority and apologise for the delay to our customers."
BA has experienced two high-profile IT failures in recent years.
In August last year, more than 100 flights had to be cancelled and a further 200 were delayed after an IT glitch involving two separate systems, one dealing with online check-in and the other with flight departures.
The airline also suffered a major computer failure over the spring bank holiday weekend in May 2017, which saw 726 flights cancelled and tens of thousands of passengers left stranded.