The Royal Marines are ready to lead a "Dunkirk-style" evacuation of thousands of Britons from the Middle East if Iran attacks Israel, according to reports.
Commandos have already carried out reconnaissance missions along the Lebanese coast.
Defence Secretary Grant Shapps said he "stands ready" to aid UK nationals stuck in the region.
The news came as the US blocked travel outside a restricted zone for all its staff in Israel amid growing fears of an imminent major attack by Iran.
As diplomatic efforts intensified to prevent hostilities escalating, the US embassy in Israel said staff were told not to venture outside the greater Jerusalem, Tel Aviv or Beersheba areas.
US President Joe Biden warned that Iran was threatening to launch a "significant attack".
Mr Shapps said: "UK nationals in the region will be concerned by what Iran is threatening to do. So we stand ready to assist them. There are plans, but they cannot be discussed for security reasons."
The evacuation plans, codenamed Operation Meteoric, will be carried out by the Green Berets in conjunction with the Royal Navy and Royal Air Force.
Marines from 30 Commando have headed to the Middle East.
The operation has been compared to Operation Dynamo, the evacuation of hundreds of thousands of allied soldiers from Dunkirk during 1940 when surrounded by German forces.
On Friday night the US said it was "moving additional assets" to the Middle East. A defence official said it aimed to "bolster regional deterrence efforts and increase force protection for USforces".
They added: "That means more military hardware is being moved or will be moved to the region".
An Iranian news agency said all airspace over Tehran had been closed for "military drills", before deleting the post on X and later denying it had been published.
US state department spokesman Matthew Miller would not disclose the "specific assessments" behind the travel restrictions, but added: "Clearly we are monitoring the threat environment in the Middle East andspecifically in Israel."
The UK Foreign Office also updated its travel advice, warning that Israel has raised the "possibility of an attack on Israeli territory from Iran, and that such an attack could trigger wider escalation".
Foreign Secretary Lord Cameron phoned his Iranian counterpart to urge against any further escalation.
Iranian-backed groups have increased attacks across the region since Israel launched its assault on Gaza in response to a Hamas attack on October 7 that killed 1,200 people.
Senior Iranian military figures were among the 13 killed in the attack in Damascus on April 1.
Israel has not confirmed it was behind the missile strike on Damascus, but the Pentagon has said it was. Iran's Supreme Leader Ayatollah Ali Khamenei said Israel "must be punished and it shall be" for theattack.
Mr Biden said US support for Israel remained "iron-clad", adding: "We will support Israel. We will helpdefend Israel and Iran will not succeed."
His advice over military action against Israel was simply, "don't."
