Passengers should continue to check with their airline
Some domestic flights in and out of Cardiff Airport face fresh disruption due to volcanic ash.
Airports in parts of Northern Ireland and Scotland closed from 0700 BST on Wednesday, aviation authorities said.
They warned airports in the north-west of England and north Wales could also be affected as the cloud moved south.
Some flights between Cardiff and Glasgow, Dublin and Cork were cancelled on Wednesday. The airport advises people to check with their airline.
Several flights from Cardiff to the Irish Republic and Northern Ireland were also cancelled on Tuesday.
Services resumed at 1300 BST on Tuesday after the Civil Aviation Authority confirmed the ash had dispersed.
A CAA spokesman said later in the day: "Met Office forecasts show that levels of ash in the atmosphere over Scotland and Northern Ireland will exceed the concentrations that engine manufacturers 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 tomorrow.
"The forecasts also show that it is likely that the ash cloud will continue to move south, potentially affecting airports in the north-west of England and north Wales tomorrow."
A spokesperson for Cardiff Airport said passengers should continue to check the website and with their airline because the picture could change "very quickly."
The ash cloud is from the same Icelandic volcano that brought havoc to European air travel in April.