Royal Navy ships and RAF helicopters were on standby in the Middle East on Sunday night over fears of a regional war.
The HMS Duncan, a Type-45 destroyer, and RFA Cardigan Bay, a transport ship, were in the eastern Mediterranean, the Ministry of Defence said.
The Foreign, Commonwealth and Development Office (FCDO) said military personnel had been deployed to embassies, while families of staff at the embassy in Beirut were withdrawn for their safety.
David Lammy, the Foreign Secretary, reiterated his advice to all British nationals in Lebanon to “leave now” while they can.
In the first major foreign policy test for Labour, John Healey, the Defence Secretary, said: “Our Armed Forces will always step up to support British citizens around the world and reinforce regional stability with their professionalism and bravery.”
Saudi Arabia joined France, Canada and Jordan in calling on their citizens to leave Lebanon.
“In a highly volatile security context” French nationals were “urgently asked” to avoid travelling to Lebanon, and those already in the country should leave “as soon as possible”, the foreign ministry in Paris said.
France also urged its nationals living in Iran to “temporarily leave”.