Airports in the London area were shut overnight due to volcanic ash from Iceland spreading across the country.
Heathrow, Gatwick and London City Airport had to close until 07.00 this morning and significant delays are expected throughout the day after they re-opened.
Manchester, Leeds/Bradford and Liverpool airports re-opened after restriction enforced over the north of England were lifted.
Other main airports also re-opened this morning after a period of restrictions over the weekend.
The National Air Traffic Service said at 11.00 today: "The CAA has lifted the no-fly zone that has been affecting flights at Heathrow and Gatwick this morning.
"The decision comes following further information from the Met Office about the nature and location of the ash cloud.
"The no-fly zone remains in place in two key areas affecting operations in Northern Ireland and the Shetland Isles. Airports falling within the no fly zones include: Belfast City, Londonderry, Shetland and Orkney. All other airports are open."
Flybe had to cancel more than 150 flights across its network today and other airlines were struggling to maintain their schedules.
Aer Lingus cancelled all flights from Dublin until midday today and others from Belfast until 13.00. Cork and Shannon departures were expected to operate as normal except for services to Heathrow, Gatwick, Manchester and Amsterdam.
by Phil Davies