An ash cloud from an Icelandic volcano grounded all but emergency flights above Britain today, leaving thousands of passengers stranded.
With the high-flying ash a threat to aircraft, air traffic control company Nats closed all controlled UK airspace until at least 7am tomorrow.
But Nats dismissed any suggestion it had over-reacted, saying safety was paramount.
With the ash drifting south, Nats was due to make a further announcement around 8pm tonight about the status of UK flights up to 1pm tomorrow.
The microscopic particles which make up volcanic ash pose a threat to aircraft because they can affect visibility and get sucked into aircraft engines, causing them to shut down.
Airports across the country, which had been able to handle some early-morning arrivals and departures before the flight ban, effectively shut down from noon - and passengers were warned to expect more disruption tomorrow.
Normally about 1,300 flights and 180,000 passengers go in and out of Heathrow every day, while Gatwick airport would expect to handle 679 flights carrying 80,000 passengers. Today only 168 flights passed through Gatwick.
At Manchester airport, a total of 202 arrivals and 311 departing flights would normally operate in the period from 7am today to 7am tomorrow.
The ash, from the Icelandic mountain Eyjaffjalljokull, also caused airport and aircraft movement shutdowns in other parts of Europe including France, Sweden, Finland, Denmark and Holland.
Einar Kjartansson, a geophysicist at the Icelandic Meteorological Office, said: "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. It depends how the wind carries the ash."
It was not the first time air traffic has been halted by a volcano, but such widespread disruption has not been seen the September 11 terror attacks.
Prime Minister Gordon Brown said he sympathised with those affected by the situation but stressed that safety must come first.
Tory leader David Cameron also expressed concern for passengers, while Liberal Democrat Treasury spokesman Vince Cable was among those who had to cancel his flight plans, scrapping an election campaign trip to Scotland.
Experts stressed that the ash, in tiny particles and difficult to detect, was no danger to public health but posed a distinct danger to aircraft.
As passengers scrambled to find other means of leaving the country, former British Airways pilot Eric Moody recalled how he and his crew safely landed a 263-passenger Boeing 747 jumbo jet in Jakarta in 1982 after ash from an erupting volcano had, temporarily, knocked out all four engines.
Mr Moody, now 68, said today: "Having flown through a cloud of ash I think that air traffic control have made the right decision today."
Those stuck at airports operating no flights bemoaned their luck. Ann Cochrane, 58, a market researcher originally from Beith in Ayrshire, was trying to get home to Toronto, where she now lives.
She said: "I think I might cry," she said. "I just wish I was on a beach in Mexico. We took a cab at 7.30am today and they told us about what was going on and said we should go home.
"It's not so bad for us because we're only down the road so we will just hire a car for another day, but other people live hours away."
Among others affected were about 50 members of the Great Britain Ice Hockey Supporters' Club heading for the world championships in Slovenia from Stansted.
At Heathrow, confused passengers milled around with their luggage, many queuing at customer information desks in a bid to rearrange flights.
There were also queues for payphones and the internet as people attempted to contact friends and relatives and find out more information.
Many were angry that all flights had been cancelled, with some saying the decision was an over-reaction.
But a spokeswoman for Nats said: "We certainly do not think we have over-reacted. Safety is our main priority and volcanic ash is a serious threat to aircraft.
"No one can remember a time before when controlled airspace has been closed in the UK.
"This is certainly one of the most significant instances of flight restrictions in living memory."
Airlines, insurance companies, the Civil Aviation Authority and the European Union all reminded passengers of their compensation rights following today's cancellations.
P&O Ferries reported a surge in passenger numbers following the airports' shutdown, while Channel Tunnel high-speed train company Eurostar said it was carrying increasing numbers on its London to Paris and Brussels services.
Some passengers unable to fly on domestic routes switched to rail, with train companies putting on more services on the East Coast and West Coast main line routes.
National Express laid on extra coaches.