Heathrow, Gatwick and London City airports have been shut down until 7am because of volcanic ash drifting across UK air space.
Aircraft in Scotland and Northern Ireland have also been grounded.
But restrictions at Manchester, Liverpool and Leeds-Bradford have been lifted.
Birmingham, Norwich and the East Midlands have also reopened.
Prestwick airport is no longer in the no-fly zone, but a spokeswoman said flights will remain grounded until lunchtime.
Air traffic authority, Nats, has said some flights at Heathrow could resume later this morning, but that passengers should prepare themselves for severe delays.
Arrivals at Gatwick will be restricted until at least 1pm, though some departures will go ahead.
Farnborough, Shoreham, Biggin Hill, all airfields in Northern Ireland, Scottish Western Isles, Oban, Campbeltown, Caernarfon and Aberdeen remain closed.