Most transatlantic flights leaving the UK were grounded today by a break-down in air traffic control systems.
The fault happened at Prestwick airport's air traffic control hub at 8am, causing delays at airports across the country, a spokewoman for NATS, which is responsible for the UK's air traffic management, said.
She said: "We have put restrictions on aircraft entering oceanic airspace over the north Atlantic."
All airborne aircraft arriving in the UK have been safely landed using manual systems, she added.
Nats said that London's Heathrow was the worst affected by the delays, with flights to the United States delayed by more than two hours.
But Gatwick airport was not as badly affected, added the spokeswoman.
Lilian Cassin, spokeswoman for the Irish Aviation Authority, said that there were no problems at Shannon Airport.
A spokeswoman for Dublin airport said that flights leaving and arriving there were unaffected.
A Manchester Airport spokesman said the problem did not seem to be causing delays there. "For us it seems like our most recent flights have gone - Virgin to Florida and Continental Airlines to Chicago.
"It sounds like we are okay at the moment."
He added that it was not possible to estimate the length of any delays springing up at Manchester.
A spokeswoman for BAA, the company which owns seven major UK airports including Heathrow and Gatwick said: "We understand that Nats engineers are currently working on resolving an issue affecting oceanic airspace.
"Passengers travelling to west-bound destinations in the US and Canada may experience some delays.
"We recommend that our customers speak to their airline before arriving at the airport."
She said that she could not give an estimate of delays being experienced at any of the company's airports.
A statement from NATS said: "At 0800 this morning, a fault developed within the Shanwick Automated Airtraffic System - which manages Oceanic airspace across the North Atlantic from the Prestwick centre in Scotland.
"Engineers are investigating the cause of the fault and working to restore the system as soon as possible.
"NATS has imposed restrictions on aircraft entering Oceanic airspace. NATS has reverted to manual systems which means reduced capacity leading to some delay. There is no safety implication.
"NATS regrets any inconvenience this may cause to travellers."