SAS soldiers were in Lebanon last night in a frantic attempt to establish how many UK nationals wish to leave.
Orders for a final evacuation could be given in as little as three days, after increased tensions caused by the killing of Hezbollah leader Hassan Nasrallah in an Israeli airstrike.
Brits have already been warned to leave the country by commercial flights.
But for the past week, British Special Forces soldiers have been liaising with the Lebanese Army to identify assembly areas for evacuees and departure points at airfields and on the coast.
Last night members of Colchester-based 3 PARA and Royal Marines from 45 Commando, based in Arbroath, were in Cyprus awaiting orders from the Permanent Joint Headquarters to move in to Lebanon.
They will be joined by troops from the Rifles and the Princess Wales’s Royal Regiment, which are already based in Cyprus.
Their task will be to mount Operation Meteoric, an evacuation by air, land and sea working in a multinational force including  US, French and Italian troops who have already arrived in Cyprus.
A further British 200 soldiers from the Special Forces Support Group and the Army Rangers regiment were last night poised to establish a cordon of protection around the main force of British troops mounting the evacuation, codenamed
Specialist bomb disposal teams, military police medics, air traffic controllers and a team from the border  force are being flown to the sovereign base.
Significantly, British troops will be armed amid fears of attacks by Hezbollah.
The terror group has already warned that foreign forces will use the pretext of an evacuation to establish another multinational bridgehead, as they did in 1982.
Troops were last night being briefed on their immediate role, which will be to establish processing areas where evacuees can be searched and their passports validated by border control staff.
A seaborne option will use  French, Italian, US and British warships to provide screen protection for the Royal Navy support ship RFA Mounts Bay and the assault ship USS Wasp, which will load fleeing personnel directly from the beaches of Beirut.
RFA Mounts Bay, a 16,000-tonne landing ship, carries special pontoons which can be powered to a beachhead and then ferry as many 100 evacuees at a time to the ship – Â while the 41,000- tonne USS Wasp can use its hovercraft and landing craft.
If a seaborne operation is not possible, fixed wing military transport planes will fly into Beirut international airport from Cyprus for a ‘Kabul style’ evacuation of what could be as many as 25,000 Europeans.
The RAF has flown four A400M transport planes forward to Cyprus, while France, Italy and the US have pre-positioned C-130 Hercules planes.
Regional expert Megan Sutcliffe of Sibylline strategic risk group, said: “Hundreds  of UK troops have now been deployed to Cyprus. Given the tempo of Israeli attacks on Lebanon, I expect to see UK forces give the order for a final evacuation within just a few days.”