A new ash cloud dashed hopes that air travel could begin to return to normal today as the row over the grounding of planes intensified.
Based on Met Office information, air traffic control company Nats allowed a handful of flights in parts of Scotland, Northern Ireland and north east England.
Nats said the restriction on flights in the rest of England and Wales would remain until at least 1am tomorrow.
Prime Minister Gordon Brown and Transport Secretary Lord Adonis stressed that passenger safety was paramount when it came to decisions about flights.
But London Mayor Boris Johnson said he was worried the shutdown of most of the UK's airspace could go on "indefinitely" and former Ukip leader Nigel Farage lashed out at EU rules for "needlessly" grounding flights.
And travel company Thomas Cook also expressed concerns about the grounding of flights.
Mr Johnson said: "What I would really like to know is whether we are absolutely certain that the initial decision taken to close down UK aviation at this level of risk was correct and whether it was capable of being revisited or not.
"I am listening to voices in the aviation sector who say they are well used to flying in volcano zones and they well understand the risks associated with plumes of volcanic ash.
"But if it is genuinely impossible to fly and if we have this long, unpredictable period of volcanic activity in Iceland ahead of us, then what plans are we making for the long-term and what are we going to do if effectively we have to disinvent aviation?"
Mr Farage said the Single European Sky policy was preventing British authorities from taking their own decisions.
"The volcano ash is a problem, but the frenzied desire of the EU for more centralised control over the aviation sector is an even greater one," he said.
As the first flights in the UK for several days arrived and took off, the struggle to bring home the estimated 150,000 Britons stranded abroad continued.
Royal Navy vessel HMS Albion sailed to Santander, in northern Spain, to pick up around 280 stuck holidaymakers as well as around 450 troops.
Another vessel - the new £500 million Celebrity Cruises ship Celebrity Eclipse - was due to leave Southampton tonight to pick up stranded Britons in Bilbao, northern Spain.
Some airlines and tour operators arranged special flights to at least get stricken Britons back to mainland Europe, while Eurostar put on extra trains on its Paris and Brussels to London routes.
But the Foreign Office warned: "Given the likely numbers involved, it will take a matter of weeks before everyone can be repatriated."
Those Britons stranded in Europe were being advised to make their way to the French port of Calais, other Channel ports or a northern European port.
For those outside mainland Europe, the Government is working with the Spanish authorities to set up an "air hub" in Madrid.
Manny Fontenla-Novoa, chief executive of travel group Thomas Cook, said: "Throughout Europe we are working round the clock to bring our customers home and get our flying programmes back to normal as quickly as possible.
"I urge the British Government to act in a coordinated way to achieve alignment with other European governments on airspace rules and a rapid return to normal flying throughout Europe."
Coaches were allowed on some ferry crossings by companies which would not normally accept foot passengers, while domestic rail companies laid on longer, and extra, trains.
Liberal Democrat leader Nick Clegg said he hoped his children, Antonio, eight, Alberto, five, and one-year-old Miguel, would be able to fly home tomorrow after being stranded in Spain where they were staying with their mother's family.
Also stuck abroad was Radio 1 DJ Chris Moyles, who had to broadcast his morning show today from a studio in New York.
There were concerns for the travel plans of fans of Fulham and Liverpool football clubs in the away legs of the semi-finals of the Europa League on Thursday. Liverpool face Atletico Madrid in the Spanish capital on Thursday, while Fulham travel to Germany to take on Hamburg.
Some Britons anxious to get home got as far as Barcelona only to find a strike prevented them travelling through France by train.
Foreign Secretary David Miliband made a direct appeal to Britons stranded across the world to make sure airlines provided them with accommodation and food.
He said more than 100 coaches would play a part in the Madrid "hub" rescue plan.
The Unite union warned the massive disruption would lead to jobs being cut in the airline industry.
British Airways had hoped to operate some flights from Heathrow today but had to scrap its plans when southern England airspace remained closed.
BA did manage to run around 12 inbound long-haul services, including one from Beijing, while airline bmi held out hope of better things by announcing it would restart Heathrow flights from 12 noon tomorrow.
Ryanair said it would not operate any flights in northern Europe until 1pm on Thursday.
The Irish budget carrier said the disruption was costing it around six million euros (about £5.25 million) a day.