Heathrow passengers spent Tuesday night sleeping on the terminal floor and a further 177 flights were cancelled on Wednesday 9 September, as the airport worked through a backlog left by Tuesday's UK-wide air traffic control failure.
Flight-tracking site Flightradar24 said 177 flights had been cancelled by Wednesday morning, "nearly all at London-Heathrow," according to CNN. That followed roughly 1,300 cancellations across UK airports on Tuesday, when a fault in the flight-processing system run by NATS, the country's air traffic control provider, halted departures at Heathrow, Gatwick, Manchester, Birmingham and other airports for hours.
Heathrow said on Wednesday morning that flights "are operating," but warned disruption could continue as airlines repositioned aircraft and crews that were left out of place overnight, CNN reported.
For some travellers, that meant an unplanned night at the airport. ABC News spoke to Perth couple Monique and Frank Whiting, who slept on the floor at Heathrow after their Icelandair flight to Iceland was cancelled following a four-hour wait on board the aircraft. "We're all staying here overnight and not getting any sleep," Ms Whiting said. Icelandair told passengers to arrange their own accommodation and offered partial reimbursement, but nearby hotels were already full, she said.
Another passenger, Debbie Payne, told ABC News she reached Heathrow for an evening flight home to Australia via Singapore only to find it had already been cancelled, with some travellers left stranded on aircraft without ramps to disembark.
Travel journalist Simon Calder said Wednesday's knock-on effects were "going to be horrible," warning of "dozens of cancellations in the morning of flights coming into Heathrow because, basically, the flights didn't go out tonight," MyLondon reported.
NATS has said it fixed the underlying technical fault on Tuesday evening but that a full return to normal operations would take longer, and chief executive Martin Rolfe apologised to affected passengers, telling the BBC there would be a full investigation, according to ABC News.
What this means for your journey: if you are flying from Heathrow in the coming days, check your flight status directly with your airline before setting off, since knock-on delays and cancellations from Tuesday's fault are still working through the schedule. If you are meeting a delayed or rebooked passenger, see our /taxis/ page for pickup options at each terminal.