Britain and Ireland were closing parts of their airspace Wednesday after a fresh cloud of ash arrived from the Icelandic volcano that caused air travel chaos in Europe last month.
British regulators ordered an airspace shutdown over parts of Scotland and Northern Ireland from 7:00 am (0600 GMT) for 12 hours for most affected airports, warning that high ash levels could damage plane engines.
"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," said Britain's Civil Aviation Authority (CAA).
"Unfortunately, this means that the CAA anticipates all Scottish and Northern Ireland airports will be closed from 07:00 local time (Wednesday)."
According to meteorologists, the cloud over Britain had "increased in density as ash emissions from the Icelandic volcano... have become stronger," said the air watchdog in a statement.
Irish aviation chiefs meanwhile said restrictions would be introduced at some airports from 8:00 am (0700 GMT).
The new shutdowns followed a closure of Irish, Northern Irish and some Scottish airspace for several hours Tuesday, which caused the cancellation of hundreds of flights and travel misery for thousands of passengers.
Airspace across Europe was closed for up to a week last month after the eruption of Iceland's Eyjafjoell volcano, but was re-opened after emergency talks between European governments, airlines and regulators.
The CAA said some Scottish airports, including Glasgow, would likely be closed for 12 hours until 7:00 pm (1800 GMT).
In Northern Ireland, Derry airport would shut down early Wednesday for 12 hours, and airports in Belfast would only close later in the day, said the air safety watchdog.
Other airports were also at risk of closure, including Edinburgh and some in the northwest of England, said the regulator.
But regulators did not indicate that the ash was an imminent threat to airports further south, including London Heathrow, Europe's busiest air hub.
In Ireland, restrictions would be brought in from 8:00 am (0700 GMT) at airports in the northwest, followed by Dublin at 11:00 am (1000 GMT).
After a cloud from the Iceland volcano last month caused the biggest aerial shutdown in Europe since World War II, ash once again grounded flights in British airspace late Monday.
Aviation chiefs grounded flights over the Outer Hebrides, a group of islands off Scotland's northwest coast, before extending the flight ban to Northern Ireland for several hours early Tuesday.
The Irish Aviation Authority also grounded flights into and out of Ireland for several hours Tuesday.
Both air authorities cleared flights to resume from 1:00 pm (1200 GMT), but new restrictions for Wednesday were announced just several hours later.
Tuesday's airspace closures came on the same day European Union transport ministers met in Brussels to discuss last month's shutdown as the new ash cloud hovered over Ireland.
A notable absence was Irish Transport Minister Noel Dempsey -- unable to fly to the meeting because of the new ash cloud chaos.
Meanwhile Irish airline Aer Lingus said the flight ban last month had cost it about 20 million euros (26 million dollars), while warning that "the final cost will depend on the actual level of customer claims."
The Association of British Insurers estimated Tuesday that the travel chaos caused by the ash had cost insurers around 62 million pounds (94 million dollars, 72 million euros).
Eurocontrol, the continent's air traffic control co-ordinator, said more than 100,000 flights to, from and within Europe had been cancelled between April 15 and 21, preventing an estimated 10 million passengers from travelling.
Explore further: Iceland's volcanic ash halts flights across Europe