The National Air Traffic Services, which oversees British airspace, said the situation from the cloud is getting worse.
Planes remain grounded until at least Sunday morning.
Volcanic ash continued to disrupt air travel across Europe Saturday. UK air traffic control has warned the situation is likely to worsen while German airspace will remain closed until at least 8 am Sunday.
European flights remained grounded on Saturday due to a vast cloud of volcanic ash from Iceland that has covered much of Europe and has spread as far east as Russia.
Britain, Ireland, Switzerland, and Belgium all extended their flight bans on Saturday. More than 15 European countries have grounded flights because of the ash cloud.
The disruption has hit German Chancellor Angela Merkel, who is trying to return to Berlin from the United States. Her plane was diverted to Lisbon on Friday and she will now fly on to Rome.
In addition, flights bringing home the bodies of German soldiers killed in Afghanistan and soldiers wounded there have been delayed.
The last time the volcano erupted, in the 19th century, it continued sporadically for over a year
Things could get worse
Volcanic ash can terminally damage the engines and windows of jetliners.
The National Air Traffic Services experts tracking the volcano beneath the Eyjafjallajokull glacier said there were no signs of reduced activity on Friday. After a relatively minor eruption one month ago, the volcano erupted again on Wednesday, with these two incidents breaking almost 200 years of dormancy.
That last recorded eruption continued – with varying levels of severity and intermittent periods of inactivity – for almost 18 months between 1821 and 1823.
“We have no prediction on how long [the eruption] will last,” said geophysicist Bergthora Thorbjnardottir at the Iceland Meteorological Office.
Hundreds of thousands stranded
Westerly and north-westerly winds have blown the ash across swathes of northern and central Europe, grounding some 17,000 flights on Friday, after around 6,000 were cancelled on Thursday.
The disruption is costing airlines over 148 million euros ($200 million) per day.
Currently, only emergency flights are permitted into and out of the affected regions in Europe.
All 16 international airports in Germany were closed on Saturday. German airspace will remain closed until at least 8 am Sunday.
Train operators and others are getting extra business
Germany’s national rail service, Deutsche Bahn, has reported a surge in demand as travellers look for another way to reach their destination.
“We have a lot more people booking tickets, and are putting every train at our disposal into action,” a Bahn spokesman told German public broadcaster ARD. “Everything that can run is running.”
The Eurostar train service linking Britain and mainland Europe via the Channel Tunnel is fully booked, with the company reporting 38,000 travelers on Friday, 10,000 more than average. International coach services also claimed to be running at five times their usual capacity due to a surge in demand.
Rental car companies reported a similar phenomenon, with some passengers even electing to drive long journeys to foreign destinations. A number of agencies in Germany were forced to turn would-be customers away.