Thousands of British citizens are preparing for the start of a mass evacuation process from Lebanon.
Thirty-seven Britons have already been evacuated by helicopter
The Foreign Office said Britons are to be evacuated in order of need, with the elderly and children going first.
It comes after Foreign Office minister Kim Howells said the operation to rescue more than 22,000 people would be the largest evacuation since Dunkirk.
Two British warships are on hand to help with the evacuation, with four more on their way to the area.
Mr Howells told MPs the UK was responsible for 12,000 Britons in Lebanon, 10,000 with dual nationality and some Commonwealth nationals for whom Britain was responsible.
A Foreign Office spokesman said "it was expected" that "the military evacuation process" would start on Tuesday.
However, it is unclear what form this process will take.
The full evacuation will follow an initial helicopter flight which took 37 of the most vulnerable British citizens to Cyprus on Monday, as Mid-East crisis worsened.
HMS Gloucester and HMS York are already in the area, with HMS St Albans, HMS Illustrious, HMS Bulwark and the Royal Fleet Auxiliary Fort Victoria expected to arrive by noon on Thursday after being sent from other parts of the Mediterranean.
A group of RAF Chinook helicopters has also been sent from RAF Odiham in Hampshire to Cyprus in case they are needed for the evacuation.
A reception centre for those evacuated is being set up in Cyprus.
British military bases in Cyprus have offered to help evacuees of all nationalities ferried to the island by providing short-term shelter as they wait for repatriation flights.
BBC correspondent Tabitha Morgan, in Cyprus, said there could be as many as 80,000 people arriving on the island in the coming days.
This would be a massive logistical exercise during the height of the tourist season with hotels already full and airports busy, she said.
Earlier, a Ministry of Defence spokesman said an evacuation would not take place unless the atmosphere was "benign". At the moment, he said, both air and land evacuation routes had been "discounted", although this could change.
Mr Howells told BBC News all Lebanese ports were blockaded, so an evacuation could only happen from a port such as Beirut with Israeli and Lebanese co-operation.
Tony Blair said the UK government was doing "everything" it could to evacuate as quickly as was possible.
It was also looking into setting up an "air bridge" into Beirut, he said, but "that is complicated for very obvious reasons".
Mr Blair and UN Secretary General Kofi Annan have called for an international force to be sent to Lebanon to stop attacks on Israel.
The force could "stop the bombardment coming over into Israel and therefore gives Israel a reason to stop its attacks on Hezbollah", Mr Blair said.
UK FOREIGN OFFICE ADVICE
Advice to British nationals in Lebanon is to stay put but be ready to move at any time
British and dual nationality citizens should call the Foreign Office on 0207 008 1500or the embassy in Beirut on (00) (961) (1) 990400
Two Royal Navy ships heading to the Middle East on stand-by for rescue operation
Britons in Israel should avoid all but essential travel areas within 25km of northern border
Israel launched its air offensive last week following the capture of two of its soldiers by Lebanese guerrillas. Hezbollah has been firing hundreds of missiles into northern Israel.
At least 180 Lebanese and 24 Israelis have been killed since fighting began on Wednesday.
British ambassador to Lebanon James Watt advised citizens waiting for evacuation to "continue staying safe, staying at home, keeping their heads down, staying in touch with us while we process the lists of who needs to go and who needs to go fastest and most urgently".
Other countries including Spain, Italy and Greece have already begun removing their nationals from Beirut, the Lebanese capital.
Roads to the east and north of Beirut, and the airport runways, have been targeted by Israeli air raids, so no flights can go in or out.
Britons in Lebanon have been contacting the BBC News website.
Schona Jolly, a human rights lawyer from London who is in a hotel in the small town of Biblos, just north of Beirut, said the situation was "extremely grave indeed".
"Me personally, I'm not afraid because I feel that we will be all right," she said.
"But I'm travelling with a friend who has a very young baby and she is of course extremely concerned at this stage.
"We go to sleep hearing Israeli jets overhead - we can hear bombing and we can see smoke."
Food was beginning to run out, she added.
Meanwhile, British former hostage Terry Waite - held captive by Hezbollah in Beirut for almost five years - has said Israeli attacks on Lebanon will "strengthen the position of the extremists".
"It is going to mean that more young people who are dissatisfied and disaffected will say, 'Right, we will join the ranks now of the terrorists'," he told GMTV.
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