Three Royal Navy ships are being deployed to help bring stranded Britons home as UK airspace remains restricted.

The UK's emergency committee Cobra discussed options to address the travel chaos caused by a volcanic ash cloud and will hold another meeting later.

The three ships are heading for Spain and unspecified Channel ports.

Air traffic control service Nats has extended flight restrictions to 0100 BST Tuesday, but ministers are hopeful airspace may reopen later that day.

Transport Secretary Lord Adonis said Met Office reports of a "dramatic reduction in volcanic activity on Monday morning" meant it might be possible to open airspace on Tuesday if the reduction was sustained.

"If it's possible to start opening airports tomorrow, contingency plans are being put in place to get as many people through those airports as fast as possible," he told BBC News.

Planes were first grounded in the UK at midday on Thursday amid fears particles in the ash cloud from Eyjafjallajoekull could cause engines to shut down.

'Human consequences'

The Met Office has advised that the volcano is now less active than previously, although weather patterns continue to blow volcanic ash towards the UK.

Brian Golding, head of forecasting at the Met Office, said there were "promising signs" the wind will be blowing in a different direction by the end of next weekend.

Travel agents' association Abta said its "rough estimate" was that 150,000 Britons are currently stranded abroad, and rail and ferry services have been stretched to the limit by passengers seeking other means to return.

HMS Ark Royal will be used to help bring Britons home

Stranded Britons: Rescue plan

Following the Cobra meeting, the prime minister outlined discussions he had held with Spanish premier Jose Luis Rodriguez Zapatero about the use of airports in Spain, which retain a limited service as they are at the edge of the ash cloud.

"I talked to Prime Minister Zapatero and he has offered in principle the use of Spanish airports as a hub to bring people back to Britain," he said.

"We are now looking (at) transport arrangements that we will support as a government - coach, ferry and train - to get people either from Madrid or another Spanish airport back to Britain."

He added that HMS Ark Royal, HMS Ocean and HMS Albion would soon be deployed towards Channel ports for the effort.

"I believe this is one of the most serious transport disruptions we have faced," Mr Brown said.

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"It's got financial consequences as well as human consequences and we will do everything in our power to make sure all the arrangements are in place to help people where possible to get back home."

The BBC understands 500 British soldiers from 3rd Battalion, The Rifles are being airlifted by chartered civilian jets from Cyprus to Spain, before sailing to the UK aboard HMS Albion.

The ship, due to arrive in the Spanish port of Santander on Tuesday morning, may also carry civilians on its return.

On Sunday, 300 soldiers from the same battalion returned to the UK via coach and ferry.

Shadow transport secretary Theresa Villiers said: "It is good news for Britons trying to get home that the government are now looking to make use of the Royal Navy.

"People stranded abroad need to know all that can be done to help them is being done and this is a first step."

Shadow tourism minister Tobias Ellwood accused the government of "dithering" in its duty to help Britons stranded abroad.

MAJOR EU AIRPORTS ON 19 APRIL Heathrow - closed Frankfurt - closed after reopening on a limited basis for several hours on Sunday Paris Charles de Gaulle - closed Schiphol, Amsterdam - closed Rome - limited service Madrid - limited service

Mr Ellwood, a former pilot, suggested allocating emergency drop-off points for all UK-bound flights in Bordeaux, Marseille, Madrid and Barcelona and bringing passengers back into the UK on planes flying at a low altitude.

Liberal Democrat transport spokesman Norman Baker said the length of time British passengers faced being stranded for needed to be "urgently assessed".

In other developments:

Clouded economics

EU transport ministers are also expected to hold emergency talks by video conference on how to ease the chaos caused by the volcanic ash cloud that has paralysed air travel across Europe.

The talks come as airports and airlines have called for flight restrictions - said to be costing airlines $200m (£130m) a day - to be reviewed.

British commercial pilots' union Balpa said the industry will need the same of kind of government rescue following the eruption as some banks have had, with a number of airlines "staring bankruptcy in the face".

Tim Jeans, managing director of the airline Monarch, said that "clearly you cannot sell a ticket for somebody from say Alicante to London for £60 and pick up a £2,000 bill".

"No business could stand that and the UK airline industry is no different."

Airports Council International (ACI) Europe and the Association of European Airlines (AEA) have questioned the proportionality of the flight restrictions currently imposed.

ACI's director general Oliver Jankovek said safety was an "absolute priority" but it was important to distinguish whether there was a "genuine risk".

However, Eurocontrol, the organisation in charge of air safety in Europe, has denied aviation authorities are being over-cautious.

I was due back at university today. Some are jeopardising their degrees by missing dissertation deadlines and important exams. We have been emailing our lecturers to inform them of our situation

Alice Pegrum

Stranded in China

Live text: Volcano ash as it happens

A number of airlines, including British Airways, have said they have carried out test flights within restricted zones with no obvious damage to aircraft.

BA chief executive Willie Walsh said analysis of its test flight and those of other airlines provided "fresh evidence that the current blanket restrictions on airspace are unnecessary".

Lord Adonis said: "In light of the test results we're getting from flights in Britain and similar test flights that have been taking place in Europe, discussions are taking place between the safety authorities and the manufacturers, to see whether there can be any updating of the safety regime for operating planes."

Meanwhile, Dr Guy Gratton, head of the Facility of Airborne Atmospheric Measurement, a joint body belonging to the Met Office and the Natural Environment Research Council, said that "it's still quite a complex mixture of clear air and very worrying - but invisible -volcanic ash at all sorts of heights".

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.

Stranded Britons should contact their local embassy, high commission or consulate.

Have you been affected by the volcanic ash cloud? Are you stranded because of the airspace restrictions? Are you in Iceland? You can send your experiences using the form below:

Send your pictures or videos to yourpics@bbc.co.uk or text them to 61124 if you are in the UKor+44 7725 100 100 if you are abroad. If you have a large file you can upload it here.

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