All flights in and out of the UK and several other European countries have been suspended as ash from a volcanic eruption in Iceland moves south.
Up to 4,000 flights are being cancelled with airspace closed in Norway, Sweden, Finland and Denmark among others.
The UK's air traffic control service (Nats) said no flights would be allowed in UK airspace until at least 0700 BST on Friday amid fears of engine damage.
Safety group Eurocontrol said the problem could persist for 48 hours.
The volcano is still spewing ash and the wind direction is expected to continue bringing clouds into UK and European airspace for some time to come.
COUNTRIES AFFECTED
Airspace closed:
UK
Republic of Ireland
Norway
Partial or planned closures:
Sweden (total closure by 2000 GMT)
France (two main Paris airports plus several in the north)
Denmark (total by 1600 GMT)
Finland (northern airspace closed till 1200 GMT Friday)
Belgium (total from 1430 GMT)
Netherlands (being shut progressively)
The UK's airspace restriction was the worst in living memory, a Nats spokesman said. Some 600,000 people are thought to have been affected.
Nats suggested that the restrictions were unlikely to be lifted after 0700, saying it was "very unlikely that the situation over England will improve in the foreseeable future".
Passengers were advised to contact their carriers prior to travel.
Transport Secretary Lord Adonis said he was "closely monitoring the situation" and would be meeting with key transport officials on Friday morning.
Experts have warned that the tiny particles of rock, glass and sand contained in the ash cloud from the still-erupting volcano could be sufficient to jam aircraft engines.
ANALYSIS
By David Shukman
BBC environment correspondent
A lone plane is in the skies above Britain tonight on a research mission to investigate the volcanic plume.
The aircraft - with scientists on board - is taking measurements of the height, density and position of the ash.
There are limits to what satellites and ground observations can discern about the plume.
Its flight path was planned to take it from Oxford to Prestwick to Lossiemouth to Newcastle and then back south.
Among the researchers on board are specialists in atmospheric science.
The aircraft - a Dornier 228 - is managed by the government-funded Natural Environment Research Council. Its head of airborne research, Peter Purcell, described the plane as "extremely adaptable and capable".
"The instrumentation will allow the crew to safely monitor the atmospheric conditions as the plume is approached," he said.
The findings will be fed to Met Office to help improve forecasts for the plume's position.
The Health Protection Agency said the ash from the Eyjafjallajoekull eruption did not pose a significant risk to public health because of its high altitude.
However, the British Lung Foundation has warned people with lung conditions to keep their medication with them as a precautionary measure.
These are some of the knock-on effects:
* Eurocontrol says Germany is monitoring the situation and considering partial airspace closures
* The two main airports in Paris and many others in the north of France are closing
* There is severe disruption in France and Spain, where all northbound flights are cancelled
* Nats is due to make an announcement shortly as to the arrangements that will be in place through to 1300 BST on Friday
* British Airways offers refunds or an option to rebook after all its domestic flights are suspended
* Flybe announces it has cancelled all flights up until 1300BST on Friday and more than 25 services due to run after that.
* British sports teams have been hit by travel problems after flights were grounded
* Dozens of Leicestershire students were evacuated from accommodation in Iceland after the volcano eruption
* Singer Russell Watson calls off concert in the Irish Republic after missing the last ferry crossing from Holyhead
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