Britons have been told to immediately evacuate Lebanon, after an Israeli airstrike on a school in Gaza sparked fresh tension in the Middle East.

The Foreign Office warned as many as 15,000 UK nationals who are still in the country that it may not be able to evacuate them if a regional war breaks out.

In updated travel advice, it said that “tensions are high and events could escalate with little warning, which could affect or limit exit routes out of Lebanon”.

It comes after an Israeli airstrike hit a school in Gaza. Local officials said it was being used to shelter displaced Palestinians and 93 people were killed.

The Israel Defence Forces said the building “served as an active Hamas and Islamic Jihad military facility” and around 20 fighters were based there.

David Lammy, the Foreign Secretary, said he was “appalled” by the attack and called for “an immediate ceasefire to protect civilians, free all hostages, and end restrictions on aid”.

“Hamas must stop endangering civilians; Israel must comply with international humanitarian law,” he said.

The incident has prompted fears of fresh hostilities between Israel and Iran-backed proxies in the region, including Lebanon-based Hezbollah.

Mr Lammy has instructed his officials to begin ramping-up and stress-testing their evacuation plans over the past week in preparation for a conflict.

The Telegraph understands he has chaired several meetings in the Government’s crisis centre to go through how the operation would unfold.

He has been told that the evacuation would be fraught with difficulty and it may prove impossible to get Britons out if Israel attacks Lebanon.

Officials have war-gamed a scenario in which Israeli airstrikes immediately take out both Beirut’s airport and its port, the only major air and sea routes out.

Israel’s advanced military hardware could also cripple communications, meaning that UK citizens in the country could not be reached by text message.

In that instance the Government would look at using FM radio channels to broadcast advice and messages to those residents still trapped.

Foreign Office officials are planning to evacuate up to 15,000 people in the event of a war, though the real number is thought to be closer to 10,000.

That would make the operation similar in size to the 2021 airlift out of Kabul which was beset with difficulties and was later branded a “disaster”.

It would be four times larger than the evacuation of British citizens from Sudan last year, which went more smoothly after a safe airfield was identified.

Beirut airport would likely be one of the first targets for Israeli airstrikes, meaning that the only viable route out of Lebanon would be by sea.

In that scenario, British nationals living inland would be advised not to make their way to the coast in case they were misidentified as military targets.

Mr Lammy has stepped up diplomatic efforts to cool tensions in recent days, holding his first phone call with the new Iranian foreign minister last week.

Iran has threatened reprisals against Israel for its assassination of Ismail Haniyeh, the political leader of Hamas, while he was staying in Tehran.

British officials fear it could use its proxies to carry out that response, especially Hezbollah, which could precipitate a wider Israeli attack on Lebanon.

Mr Lammy also spoke to Israel Katz, the Israeli foreign minister, to urge restraint in Gaza, with the huge death toll there fuelling much of the regional tension.

A Foreign Office spokesman said: “While we continue to use all diplomatic levers to push for de-escalation, our staff are working around the clock to plan for all scenarios to keep British nationals safe.

“Our travel advice is constantly updated to reflect the latest guidance, but the clear message is leave now.”