Keir Starmer said the death of the leader of Hamas provides an opportunity to broker a ceasefire in Gaza.
The Prime Minister said Yahya Sinwar, who was killed by Israeli soldiers in Rafah, had the blood of Israelis and Palestinians on his hands.
Speaking after talks with world leaders in Berlin, Mr Starmer: "No one should mourn the death of Hamas leader Sinwar.
"On his hands is the blood of innocent Israelis, killed on 7 October and over years of terror. And also the blood of the Palestinian people who suffered in the chaos and violence that he sought and celebrated."
Mr Starmer said it was critical to "make the most of this moment", adding: "I do think the death of Sinwar provides an opportunity for a step towards that ceasefire that we've long called for."
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The PM met US President Joe Biden, France’s Emmanuel Macron and Germany’s Olaf Scholz on Friday in Berlin for a gathering of the “quad” group of nations.
The UK and its allies will continue to press for ceasefires in Gaza and Lebanon, the release of hostages and for aid to be allowed into Gaza. The PM said he would not widen the ban on arms sales to Israel as the UK continues to support Israel's right to self defence in the face of attacks from Iran.
But in a warning to Benjamin Netanyahu, he said: "The world will not tolerate any more excuses on humanitarian assistance. "Civilians in northern Gaza need food now."
The leaders were "absolutely united in our resolve" to back Ukraine for "as long as it takes", he said. "As Ukraine enters a difficult winter, it's important to say we are with you. We're absolutely united in our resolve and will back Ukraine for as long as it takes."
He added that Russia was "getting weaker" as the war soaks up 40% of its budget. "The only acceptable outcome is a sovereign Ukraine and a just peace," he said.