Sir Keir Starmer said the international community must “make the most of this moment” in pushing for an end to the Middle East conflict, as he met allies in Berlin after the death of Hamas’ leader.
The Prime Minister said the killing of Yahya Sinwar offered an “opportunity” to step up ceasefire talks that have so far failed to produce a breakthrough after joining US, French and German leaders on Friday.
Speaking at a press conference after discussions as part of the so-called “Quad” of nations, Sir Keir said the “answer is diplomacy” in resolving the long-running war in Gaza and violence in the wider region.
The planned trip to the German capital came after Israel announced on Thursday its military had killed Hamas leader Mr Sinwar, presenting his death as a turning point in the country’s campaign against the group.
Sir Keir said: “No-one should mourn the death of Hamas leader Sinwar. On his hands is the blood of innocent Israelis killed on October 7 and over the years of terror, and also the blood of Palestinian people who suffered in the chaos and violence that he sought and celebrated.
“Allies will keep working together to de-escalate across the region, because we know there is no military-only solution.
“The answer is diplomacy and now we must make the most of this moment. What is needed now is a ceasefire, immediate and unconditional release of all hostages, immediate access to humanitarian aid and a return to the path towards the two-state solution, as the only way to deliver long-term peace and security.”
He added that he believed “the death of Sinwar provides an opportunity for a step towards that ceasefire that we have long called for”.
Sir Keir also repeated a warning to Israel that the world will not tolerate “any more excuses” for not allowing humanitarian aid into Gaza, where more than 42,000 Palestinians have been killed in Israel’s offensive.
The Prime Minister’s words echo the message from Joe Biden, to whom he spoke on Friday, after the US President said Mr Sinwar’s killing was a “good day for Israel, for the United States and for the world”.
Officials in Washington expressed muted optimism that his death may remove a key obstacle in faltering ceasefire talks.
However, Israeli Prime Minister Benjamin Netanyahu said the country will keep fighting until all Israeli hostages are free, and that it will keep control over Gaza long enough to ensure Hamas does not rearm.
Earlier this month Israel also opened a new front in its war with Hezbollah, stepping up its bombardment of Lebanon and a ground offensive after a year of trading fire across borders.
Mr Sinwar’s death is a crippling blow to Hamas, but the group has proved resilient after the loss of previous leaders.
He became the overall leader of Hamas in July after his predecessor, Ismail Haniyeh, was killed in an apparent Israeli strike in the Iranian capital Tehran.
Following Friday’s meeting between Sir Keir, Mr Biden, Chancellor Olaf Scholz and French President Emmanuel Macron, the US leader told reporters that he and his counterparts “think that there is a possibility of working for a ceasefire in Lebanon and it’s going to be harder in Gaza”.
He continued: “But we agree there has to be an outcome of what happens the day after.”
Elsewhere in the talks, the four discussed Russia’s invasion of Ukraine, which Sir Keir suggested was leading to Moscow “getting weaker” as its war soaks up 40% of its budget.
Asked about reports that North Korea has deployed troops to assist Moscow, he said it suggested “desperation” on the part of the Kremlin.
“If this is true then to me it looks more an act of desperation than anything else,” Sir Keir said.
Allies are “absolutely united in our resolve” to back Ukraine for “as long as it takes”, he added.
“As Ukraine enters a difficult winter, it’s important to say we are with you,” Sir Keir said.
“The only acceptable outcome is a sovereign Ukraine and a just peace.”
