Two Royal Navy ships are expected to make their way to the Middle East to be on standby for an emergency evacuation of British nationals from the Lebanon.
Defence Secretary Des Browne has given orders for HMS Illustrious and HMS Bulwark to "make ready" for operations following Israeli air strikes.
No order has yet been given to evacuate UK citizens from the region.
But British citizens have been warned to prepare for a departure at short notice if the situation changes.
The ships will leave as soon as necessary, possibly within 24 hours.
Israel stepped up its strikes in Lebanon after Hezbollah militants seized two Israeli soldiers.
HMS Illustrious is currently in Gibraltar and HMS Bulwark is close to Barcelona in Spain.
Foreign Secretary Margaret Beckett said British nationals in Lebanon should stay put, make sure the embassy knows of them, keep a low profile and take embassy advice as it becomes available.
A Foreign Office spokesman added: "We are also advising British nationals to get ready for departure at short notice if the situation changes including by having travel documents in order."
Meetings on the evacuation plan were taking place at the Ministry of Defence.
But the decision whether to proceed with an evacuation plan has not yet been taken.
Britons with relatives in Lebanon told the BBC News website they feel let down by politicians and diplomats.
Rushee Jolly, whose 31-year-old sister Schona is stuck in the country, said the government "has to do something to protect their nationals".
"We feel that the embassy in Beirut is doing nothing, and that the advice they are giving is pretty poor.
"They told her to sit tight when the place where she was was being shelled."
Peter Jones, whose 16-year-old son Perry is spending his school holidays near the northern Lebanon city of Tripoli, said he was "worried to death".
"They have been seeing war-planes flying over, and the Israelis have been bombing Tripoli.
"When the planes get too close, they go into the bomb shelter they built in the basement of their house."
He said the embassy had not given them any advice on how their son could leave the area.
Israeli Prime Minister Ehud Olmert has said the offensive against Lebanese targets will continue until Hezbollah releases the soldiers and stops firing rockets at Israel.
There are several thousand Britons in Beirut. One who has got out of the city is student Angela Quartermaine, from Oxford.
Referring to the news that the two Royal Navy vessels were on standby, she said: "I don't know how they are going to get here as the port has been bombed... we'll believe it when we see it."
BBC defence correspondent Paul Wood has said that 24 hours to get ready for sailing would be considered a "short turnaround" for the two ships.
He said that damage to the airport and port in Beirut meant it would be "difficult to get in".
"Any evacuation would be carried out in concert with other countries, led by the US," he added.
Mr Wood said HMS Bulwark had a contingent of 500 marines, which could be used to make beaches safe in the event of any evacuation.
HMS Illustrious has helicopters and fighter bombers on board, which again could be used to aid any evacuation.
British and dual nationality citizens contacting the Foreign Office in London should call 0207 008 1500.