RAF helicopters were ready for a major evacuation as the Prime Minister urged British nationals in Lebanon to “leave immediately” as fighting between Israel and Hezbollah continues to escalate.
The UK is sending around 700 troops to Cyprus in case an emergency evacuation of the country is required.
Royal Navy ships RFA Mounts Bay and HMS Duncan are deployed in the eastern Mediterranean for an evacuation mission.
RAF transport aircraft and helicopters are also on standby.
Airlines suspending flights to Lebanon because of the escalating violence means commercial escape routes could be cut off as the situation deteriorates.
The Prime Minister will be at the United Nations in New York on Wednesday and Thursday as world leaders attempt to prevent the fighting in Lebanon spiralling into an all-out war.
Speaking on his plane to the US, he said: “The most important message from me this evening is to British nationals in Lebanon, to leave immediately and I just want to reinforce that.
“Yes, we are ramping up the contingency plans, I think that you would expect that in light of the escalation.
“But it is important that we be really, really clear: now is the time to leave.
“More broadly, I am worried about the situation and I think we need to be clear we need de-escalation, we need a ceasefire, we need to pull back from the brink.
“I think that will be amongst the first topics we discuss in New York.”
Former UK National Security Adviser Lord Ricketts stressed that British armed forces had experience in carrying out an evacuation from Lebanon, with preparations now being stepped up.
“This is sensible contingency planning,” he told Times Radio.
“We are clearly near the point now where commercial flights are going to stop.
“Obviously, it’s best for British people to leave on commercial flights, they are much more effective at moving large numbers of people than the military.
“But the military are well prepared now to do an evacuation if they need to do it.
“We did an evacuation in 2006 involving helicopters and aircraft and then ships to move I think 4,000 people out of Lebanon at the height of the Israeli incursion in 2006.
“So this is something that the military are well practised at doing and they are now set up to do it if the airport should close and people simply can’t get out.”
Israel killed a top Hezbollah commander on Tuesday as part of a two-day bombing campaign that left more than 560 people dead and prompted thousands in southern Lebanon to seek refuge from the widening conflict.
With the two sides on the brink of all-out war, Hezbollah launched dozens of rockets into Israel, including a longer-range projectile that set off air raid sirens in Tel Aviv and across central Israel.
It was the Iran-backed group’s furthest strike yet in nearly a year of exchanges. Israel said it intercepted the projectile, and there were no reports of casualties or damage.
The British deployment to Cyprus comes as the Government begins the first stage of its Dunkirk-style contingency plan, with the military team supported by Border Force and Foreign Office officials.
The Royal Air Force has planes and transport helicopters on standby to provide support if necessary.
Defence Secretary John Healey announced the movement of troops on Tuesday night following a Cobra meeting earlier in the day.
Mr Healey said: “Events in the past hours and days have demonstrated how volatile this situation is, which is why our message is clear, British nationals should leave now.
“We continue to urge all sides to step back from conflict to prevent further tragic loss of life.
“Our Government is ensuring all preparations are in place to support British nationals should the situation deteriorate.
“I want to thank the British personnel who are deploying in the region for their commitment and professionalism.”
The UK already has a significant diplomatic and military presence in the region, including RAF Akrotiri in Cyprus and British ships RFA Mounts Bay and HMS Duncan, which have remained in the eastern Mediterranean over the summer.
The Cobra meeting chaired by Mr Healey was attended by intelligence chiefs and diplomats and provided an opportunity to test government planning.
The need for a contingency plan was underlined as more airlines cancelled flights to Lebanon on Tuesday.
Airlines in the United Arab Emirates, a key East-West travel hub, cancelled flights on Tuesday.
Long-haul carriers Emirates and Etihad cancelled flights, as did FlyDubai, the low-cost carrier.
EgyptAir also cancelled flights to Lebanon.
