(CNN) -- The air traffic control system controlling London airspace has been restored after a technical failure, the UK air traffic management company NATS said Friday, but travelers can expect disruption and cancellations to flights.
"We are in the process of returning to normal operations. We apologise for any delays and the inconvenience this may have caused," NATS said.
"Further information will be released as it becomes available."
NATS said a short time before that London airspace was open but that traffic was being restricted in line with the capacity in the system.
Travelers passing through London can expect to experience delays or disruption to their flights for hours to come.
Heathrow Airport said the NATS systems were "now working and stable" but that there are still some restrictions to airspace.
The airport, the third-busiest in the world in terms of passenger traffic, warned that disruption could continue into Saturday.
"The earlier problem will cause delays and cancellations to flights for the rest of the day and is likely to have a knock on effect on some services tomorrow because aircraft and crew will now be out of position," it said in a statement.
"Passengers due to depart today and tomorrow should check the status of their flight with their airline before travelling to Heathrow. We are very sorry for the disruption to passengers' journeys."
Extra staff will be on duty to help passengers, it said.
A spokesman for budget airline Ryanair, Robin Kiely, told CNN that flights were expected to resume with delays but that cancellations were not expected.
He slammed NATS over the problem, saying it was "unacceptable" that its system had "dropped for the second time in 12 months, particularly on a busy Friday in the run up to Christmas."
British Airways offered refunds or the chance to rebook to any of its passengers not wishing to travel Friday in light of the problems.
"We anticipate disruption to both departing and arriving flights but will do all we can to minimise any impact," it said in a statement.
It's not just UK travelers who will be affected by the glitch.
"All flights that are supposed to fly London from Charles de Gaulle are delayed," a spokesperson for Paris Charles de Gaulle airport told CNN.
"Flights from Tunisia will be landing at Charles de Gaulle instead of London."
Mary Ryan, a spokeswoman for United Airlines, said none of its inbound flights Friday was affected but that four outbound flights were being held at the gate until the issue was resolved.
A spokesman for Delta Airlines advised passengers bound for London Friday to check the status of their flight before traveling.
The European air traffic control network, Eurocontrol, earlier said all London airspace was closed because of a computer failure, and that no flights would be accepted in or out of London until 7 p.m. (2 p.m. ET.)
The issue affected the air traffic control center in Swanwick, which controls all air traffic routes in southern England and Wales, up to approximately Manchester. A second air traffic control center at Prestwick in Scotland takes over air routes from there. It has not been affected, NATS said.
Thousands of travelers heading into or out of the British capital for the weekend are likely to be delayed, as well as those on flights connecting through London airports. Besides Heathrow Airport, they include Gatwick, Stansted, Luton and London City airports.
CNN's Alexander Felton, Karl Penhaul, Rachel Kitchen and Richard Allen Greene contributed to this report.