The disruption has affected hundreds of thousands of travellers
Britons stranded by flight restrictions as a result of the volcanic ash cloud from Iceland could be returned to the UK through a "Spanish hub".
Ministers met to discuss plans before UK flight restrictions were extended until at least 1900 BST on Monday.
Ideas included flying those outside the no-fly zone to Spain and then using the Royal Navy and requisitioning merchant ships to help return them to the UK.
Gordon Brown has spoken to his Spanish counterpart about the plan, No 10 said.
A spokeswoman said the prime minister and the Spanish leader had explored whether Britons could be returned by landing in Spain - which is open to flights - from certain parts of the world.
A spokeswoman said: "They agreed that the UK and Spain would work together to look at how the capacity in Spain... can be used to help get British people stranded abroad back to the UK.
"Prime Minister Zapatero said that he would help in any way he can."
Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, Germany, Hungary, Ireland, Latvia, Luxembourg, Netherlands, Poland, Romania, Slovakia, Switzerland, UK
Italy (northern airspace closed until Monday)
Norway (most airports open)
Bulgaria (Sofia and Plovdiv open)
Poland (several airports, including Warsaw, open)
Sweden (northern airports open)
France (southern airports open)
Greece, Lithuania, Portugal, Russia, Turkey, Ukraine, Spain
But she said it was too early to say if additional flights would be added to Spain-bound schedules.
Earlier, Business Secretary Lord Mandelson had emerged from Sunday's 85-minute meeting of ministers flanked by several cabinet colleagues and said: "We will mobilise all possible means to get people home."
Travel agents' association Abta said on Sunday its "rough estimate" was that 150,000 Britons globally had been unable to return to the UK because of the flight restrictions first imposed on Thursday.
"At no time in living memory has British airspace been shut down and affected this many people," a spokeswoman said.
Forecasters have warned the dust cloud generated by the Icelandic eruption may remain over the UK for several days.
The continued ban on UK flights comes as bodies representing European airports and airlines have called for flight restrictions to be reviewed and a number of airlines, including BA, have carried out test flights.
BA test flight
Planes were first grounded in the UK at midday on Thursday amid fears that particles in the ash cloud could cause engines to shut down.
Transport Secretary Lord Adonis said data from a number of test flights would go to regulators and there would be a meeting of European transport ministers on Monday.
Additional capacity had been introduced on other transport such as Eurostar and Eurotunnel trains, and ferries, he added.
AT THE SCENE
Lorna Gordon, BBC News, Iceland
In some small areas the volcanic fallout has been significant. It is clogging car engines, turning grass grey and reducing visibility to just a few metres.
The police say driving conditions can be very difficult in these places. I heard one tale recounting that the moment you drive into the ash cloud it can feel as if you are driving into a wall.
The affected area is remote with only a few hundred people, most of them living in isolated homes and many of them farmers. They have been advised to stay inside with the windows and doors shut and if they do venture out to wear goggles and a mask.
Despite the hazards the volcano and its column of smoke are drawing visitors. They are also triggering lightning. The authorities are having to remind people they should not consider the volcano a tourist attraction. In fact, they have closed the country's ring road in the area affected to all but locals and the emergency services.
Security minister Lord West, a former head of the Royal Navy, said using the navy to bring people home was an option.
The government's Cobra emergency committee is to meet at 0830 BST on Monday.
The Conservatives have released an eight-point plan they would like to see to tackle the situation.
It includes chartering ships to bring people home who are stranded in Europe and urging ferry and rail operators to retain their normal pricing structures.
Shadow transport secretary Theresa Villiers said: "With thousands of Britons stuck in airports overseas, it is hugely worrying that there is no end in sight for the flight ban."
Liberal Democrat transport spokesman Norman Baker said it needed to be "urgently assessed" how much longer British passengers faced being stranded for.
Meanwhile, a British Airways Boeing 747 had completed a two-and-three-quarter hour test flight at 40,000ft from Heathrow to Cardiff, via the Atlantic, the airline said.
Chief executive Willie Walsh, who is a trained pilot, and four crew were on board.
"The conditions were perfect and the aircraft encountered no difficulties. It will now undergo a full technical analysis at British Airways' engineering base at Cardiff," the airline said in a statement.
However, BBC business editor Robert Peston said a Met Office plane had encountered dangerous levels of ash when it went through the ash cloud on Sunday.
He said this showed the issue was not whether the cloud was real and dangerous - but whether its extent could be accurately mapped.
One possible solution would involve putting observation planes in the sky to give a more detailed picture of the location of ash concentrations.
The government is trying to obtain more observation planes - from the military in particular - with this in mind, he said.
Dutch airline KLM and German airline Lufthansa have also carried out test flights in their countries' airspace to see if it is safe for planes to fly.
KLM said it had flown a plane through the cloud of volcanic ash without suffering any damage.
Air France said it had successfully carried out a test flight from Paris to Toulouse.
Airports Council International (ACI) Europe and the Association of European Airlines (AEA) said they were "questioning the proportionality of the flight restrictions currently imposed".
AEA secretary general Ulrich Schulte-Strathaus said: "Verification flights undertaken by several of our airlines have revealed no irregularities at all."
Earlier, Brian Flynn, head of operations at Eurocontrol, the organisation in charge of air safety in Europe, denied aviation authorities were being over cautious.
Ryanair has cancelled all scheduled flights between the UK, Irish Republic, Denmark, Finland, Norway, Sweden, Belgium, Holland, France, Germany, Poland and the Baltic states until 1300 BST on Wednesday.
Easyjet has cancelled all its flights scheduled before 1300 BST on Monday in areas where airspace is closed, but a limited number are expected to operate in southern Europe.
BMI said it was cancelling all its Heathrow flights until midnight on Monday.
Anyone concerned about the safety of a British national stranded abroad can call a Foreign Office helpline on 020 7008 0000, or visit its website at www.fco.gov.uk.
Stranded Britons should contact their local embassy, high commission or consulate.
The disruption has affected hundreds of thousands of travellers since Wednesday, when the Eyjafjallajoekull volcano in Iceland began erupting.
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