Delays caused by air traffic control problems could last for the entire day, an aviation expert has warned.
Independent aviation analyst Chris Yates said: "It's going to be a day of frustrations and the knock-on effects are going to last for the whole day because of the backlog of planes. It will be a tough day for everybody.
"There are contingency plans in place whenever this happens. Many of the long-haul flights, coming from China, India, the US and so on, passengers sitting on those planes may find themselves diverted to continental airports.
"But it's going to be a long wait for them. When the system kicks back in and starts working, there will be a backlog of flights.
"For those waiting to fly out, it's going to be a case of sitting around the airport terminal until things get back to normal."
More top news
It will be a mild but cloudy start for many of us on Wednesday morning, with rain in the west and drier and brighter weather in the east.
One week on from the Westminster terror attack a vigil is to be held on Westminster Bridge to remember the victims killed by Khalid Masood.
A man who was paralysed in a cycling accident can now move his right arm and hand thanks to thought-controlled technology.