Flights across much of Europe are being cancelled on a second day of massive disruption caused by drifting ash ejected from a volcano in Iceland.
Hundreds of thousands of passengers are affected and severe disruption could extend into the weekend, including on flights to North America and Asia.
Some 5,000 flights were cancelled on Thursday as airspaces from the Republic of Ireland to Finland were closed.
The ash is not thought to pose a serious health risk to people however.
Health officials in Scotland, where the ash was expected to start falling overnight, said the concentration of particles that fell was likely to be low, and effects on people with existing respiratory conditions were "likely to be short term".
The UK's Met Office said any ash that did reach ground level would be barely visible.
UK restrictions originally in place until 1300 (1200 GMT) on Friday have been extended until at least 1900 (1800 GMT), although some exceptions may be possible in Scotland and Northern Ireland.
"In general, the situation cannot be said to be improving with any certainty," said a statement issued by the National Air Traffic Service.
'Progressing very slowly'
The European air traffic control organisation, Eurocontrol, said a lack of wind meant the ash cloud created by the volcano underneath Iceland's Eyjafjallajoekull glacier was "progressing very slowly eastwards" and remained "very dense".
The airspaces of the UK, Irish Republic, Denmark, Norway, Sweden, Finland, Belgium and the Netherlands closed their airspaces on Thursday.
France shut down 24 airports in the north of the country, including the main hub of Paris-Charles de Gaulle, while Germany's Berlin and Hamburg airports were also closed on Thursday evening.
Qantas, Japan Airlines, Korean Air, Singapore Airlines and Cathay Pacific were among long-haul airlines who have cancelled flights to Europe.
"Our personal view is it may take until Sunday" for flights to resume, said Qantas spokesman David Epstein.
If the disruption persists, there are fears in Poland that some world leaders will be unable to attend Sunday's state funeral of Polish President Lech Kaczynski, who was killed in a plane crash last Saturday.
Several European monarchs were unable to attend 70th birthday celebrations for Denmark's Queen Margrethe, which began with a concert on Thursday.
For want of a plane, Swedish Foreign Minister Carl Bildt was known to be driving home to Sweden from Brussels, the Associated Press news agency reports.
'Days or weeks'
The volcano began erupting on Wednesday for the second time in a month, hurling a plume of ash 11km (seven miles) into the atmosphere. A 500m-wide fissure appeared at the top of the crater.
The heat melted the surrounding ice, and witnesses say two flows of meltwater started coming off the glacier on Wednesday.
As many as 800 people were evacuated from their homes as water carried pieces of ice reportedly the size of small houses down the mountain. A road along the flooded Markarfljot river was also cut in several places.
On Thursday, the flooding was reported to have subsided, but the volcano was still producing ash that was being blown towards Europe.
"It is likely that the production of ash will continue at a comparable level for some days or weeks. But where it disrupts travel, that depends on the weather," Einar Kjartansson, a geophysicist at the Icelandic Meteorological Office, told the Associated Press.
"It depends how the wind carries the ash."
The last volcanic eruption beneath the glacier was on 20 March. The eruption before that started in 1821 - and continued for two years.
Iceland lies on the Mid-Atlantic Ridge, the highly volatile boundary between the Eurasian and North American continental plates.
Airspace closed:
UK
Republic of Ireland
Norway
Denmark
Sweden
Belgium
Netherlands
Finland
Partial or planned closures:
France (northern airspace)
Poland (northern airspace)
--
Wonderful, some parts here also got ash. Heh, more cleaning. Thankfully I don't need planes until summer.
Hundreds of thousands of passengers are affected and severe disruption could extend into the weekend, including on flights to North America and Asia.
Some 5,000 flights were cancelled on Thursday as airspaces from the Republic of Ireland to Finland were closed.
The ash is not thought to pose a serious health risk to people however.
Health officials in Scotland, where the ash was expected to start falling overnight, said the concentration of particles that fell was likely to be low, and effects on people with existing respiratory conditions were "likely to be short term".
The UK's Met Office said any ash that did reach ground level would be barely visible.
UK restrictions originally in place until 1300 (1200 GMT) on Friday have been extended until at least 1900 (1800 GMT), although some exceptions may be possible in Scotland and Northern Ireland.
"In general, the situation cannot be said to be improving with any certainty," said a statement issued by the National Air Traffic Service.
'Progressing very slowly'
The European air traffic control organisation, Eurocontrol, said a lack of wind meant the ash cloud created by the volcano underneath Iceland's Eyjafjallajoekull glacier was "progressing very slowly eastwards" and remained "very dense".
The airspaces of the UK, Irish Republic, Denmark, Norway, Sweden, Finland, Belgium and the Netherlands closed their airspaces on Thursday.
France shut down 24 airports in the north of the country, including the main hub of Paris-Charles de Gaulle, while Germany's Berlin and Hamburg airports were also closed on Thursday evening.
Qantas, Japan Airlines, Korean Air, Singapore Airlines and Cathay Pacific were among long-haul airlines who have cancelled flights to Europe.
"Our personal view is it may take until Sunday" for flights to resume, said Qantas spokesman David Epstein.
If the disruption persists, there are fears in Poland that some world leaders will be unable to attend Sunday's state funeral of Polish President Lech Kaczynski, who was killed in a plane crash last Saturday.
Several European monarchs were unable to attend 70th birthday celebrations for Denmark's Queen Margrethe, which began with a concert on Thursday.
For want of a plane, Swedish Foreign Minister Carl Bildt was known to be driving home to Sweden from Brussels, the Associated Press news agency reports.
'Days or weeks'
The volcano began erupting on Wednesday for the second time in a month, hurling a plume of ash 11km (seven miles) into the atmosphere. A 500m-wide fissure appeared at the top of the crater.
The heat melted the surrounding ice, and witnesses say two flows of meltwater started coming off the glacier on Wednesday.
As many as 800 people were evacuated from their homes as water carried pieces of ice reportedly the size of small houses down the mountain. A road along the flooded Markarfljot river was also cut in several places.
On Thursday, the flooding was reported to have subsided, but the volcano was still producing ash that was being blown towards Europe.
"It is likely that the production of ash will continue at a comparable level for some days or weeks. But where it disrupts travel, that depends on the weather," Einar Kjartansson, a geophysicist at the Icelandic Meteorological Office, told the Associated Press.
"It depends how the wind carries the ash."
The last volcanic eruption beneath the glacier was on 20 March. The eruption before that started in 1821 - and continued for two years.
Iceland lies on the Mid-Atlantic Ridge, the highly volatile boundary between the Eurasian and North American continental plates.
Airspace closed:
UK
Republic of Ireland
Norway
Denmark
Sweden
Belgium
Netherlands
Finland
Partial or planned closures:
France (northern airspace)
Poland (northern airspace)
--
Wonderful, some parts here also got ash. Heh, more cleaning. Thankfully I don't need planes until summer.
