Heathrow and Gatwick airports have had flying bans lifted until at least 7pm, said the Civil Aviation Authority.
The decision comes following further information from the Met Office about the density and location of the ash cloud.
There are no restrictions at other English or Welsh airports during this period.
Until 1pm the no-fly zone remains in place in Northern Ireland, Edinburgh, Aberdeen, Inverness and Northern Scotland.
From 1pm - 7pm the no-fly zone remains in place in two key areas affecting operations in Northern Ireland and the Orkney and Shetland Isles.
Airports falling within the no-fly zones include Belfast City, Londonderry, Kirkwall and Stornoway. All other airports will be open.