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
We'll have to wait until the weekend but by then it will be warming up.
Facebook says it will start test flights later this year of a solar-powered drone that aims to provide remote areas with the internet.
Children's charity Kids Company is being investigated by the Metropolitan Police over criminal allegations.