Recent attacks on Israel expose Iran's "malign role" in the Middle East, the Prime Minister said on Monday.
Sir Keir Starmer said the UK would continue to support Israel’s right to self defence after Iran launched almost 200 ballistic missiles at the country in retaliation for Israeli strikes on Lebanon which killed Hezbollah leader Hassan Nasrallah.
The PM also reiterated calls for British citizens to immediately leave Lebanon in a statement to MPs a year on from the October 7 terror attacks by Hamas.
But he also urged Israel to show the "courage of restraint", as the country’s Prime Minister Benjamin Netanyahu weighs up how to retaliate against the missile attack, while Hamas and Hezbollah launched further rocket barrages.
"We support Israel's right to defend herself against Iran's aggression, in line with international law, because let's be very clear, this was not a defensive action by Iran, it was an act of aggression and a major escalation in response to the death of a terrorist leader,” Sir Keir told the Commons.
"It exposes, once again, Iran's malign role in the region: they helped equip Hamas for the seventh of October attacks, they armed Hezbollah, who launched a year-long barrage of rockets on northern Israel, forcing 60,000 Israelis to flee their homes, and they support the Houthis, who mount direct attacks on Israel and continue to attack international shipping."
He added: "I say again, an important message to British citizens still in Lebanon, you must leave now."
Most of Iran’s missiles launched last week were intercepted, but a small number struck central and southern Israel last Tuesday night.
The only person reported to have been killed was a Palestinian man in the occupied West Bank.
It follows an attack in April when Iran launched about 300 missiles and drones at Israel.
Sir Keir pledged to use the "power of diplomacy" to "minimise the suffering on the ground" in the Middle East.
"Nobody in this House can truly imagine what it feels like to cower under the bodies of your friends hoping a terrorist won't find you, mere minutes after dancing at a music festival,” he said in reference to the October 7 Hamas attacks in which 1,200 people were killed and hundreds more taken hostage.
"Nobody in this House can truly imagine seeing your city, your homes, your schools, your hospitals, your businesses obliterated, with your neighbours and family buried underneath.
"It is beyond our comprehension, and with that should come a humility.
"It is hard even to understand the full depth of this pain, but what we can do is remember.
“What we can do is respect and listen to the voices that reach out to us at these moments.
"And what we can do is use the power of diplomacy to try and find practical steps that minimise the suffering on the ground and work towards that long-term solution so that a year of this terrible and bloody conflict can never happen again."
