Three Royal Navy ships are being drafted 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.
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 the 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.
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.
"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."
HMS Albion was deployed to Spain to return 220 members of the 3 Rifles battalion to the UK, as they remain stranded on their way back from Afghanistan.
On Sunday, 300 of the 3 Rifles battalion returned to the UK via coach and ferry.
HMS Albion is due to arrive in Santander, in northern Spain, on Tuesday morning and may also carry civilians on its return.
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.
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:
- British Airways says it has asked the European Union and the government for financial compensation for the closure of airspace
- Europe's biggest travel operator TUI Travel, which owns Thomson and First Choice, says the disruption caused by the volcanic ash has cost the group about £20m so far.
- Bodies representing most European airlines and airports call for an urgent review of the flight restrictions being imposed
- Teachers' union NASUWT has urges schools not to penalise teachers who fail to return after the Easter holiday because of flight suspensions
- Norfolkline, which runs a cross-channel ferry service between Dover and Dunkirk, is providing coaches on its ferries to enable foot passengers to travel to and from the UK on Monday
- Stranded travellers are using social media to organise inventive ways to return home
I was reading earlier about a toddler whose in critical condition because a donor transplant she was due to have is delayed in Canada because of the travel disruptions. It puts things into perspective for someone like me who was worried about losing money from my holiday.