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.
