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.
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.
