Airspace over Scotland and Northern Ireland will be closed from 7am due to the Icelandic ash cloud, the Civil Aviation Authority has said.
Airports and flights in the northwest of England and North Wales could also be hit during the afternoon if the ash continues to move south.
Airspace over parts of Scotland, Northern Ireland and the Republic of Ireland is expected to be closed for up to 12 hours as volcanic ash levels in the atmosphere once again make it unsafe to fly.
The CAA said no-flight restrictions will be introduced from 7am to 7pm for Glasgow, Prestwick and Campbeltown in mainland Scotland and Stornoway, Benbecula, Tiree, Islay and Barra in the Hebrides.
Inverness airport will be shut from 7am to 1pm.
Airspace over Northern Ireland is also affected with Derry likely to be closed from 7am to 7pm and Belfast International and Belfast City from 1pm to 7pm.
In the Republic of Ireland, flights are suspended until further notice at Donegal from 8am, Sligo from 9am and Dublin and Knock from 11am.
Restrictions are also likely to be imposed on Cork, Waterford and Kerry at 2pm, the Irish Aviation Authority said.
The announcement follows disruption on Tuesday when hundreds of flights were cancelled as the Icelandic cloud returned once again to UK airspace.
A CAA spokeswoman said: “Met Office forecasts show that levels of ash in the atmosphere over Scotland and Northern Ireland will exceed the concentrations that engine manufactures have agreed are safe for operations.
“Unfortunately, this means that the CAA anticipates all Scottish and Northern Ireland airports will be closed from 7am local time.
“The forecasts also show it is likely that the ash cloud will continue to move south, potentially affecting airports in the northwest of England and North Wales.”
The CAA describes the situation as “very dynamic” and passengers expecting to travel from the impacted airports should contact their airlines to check whether their flight is operating.
Last month the ash cloud forced the closure of much of Europe’s airspace for almost a week.
