Sir Keir Starmer has assured Ukrainian President Volodymyr Zelensky the UK’s support for Ukraine in its war with Russia remains “iron-clad”.
The two men met at a summit of the European Political Community in the Hungarian capital, Budapest.
The US has been by far the largest single donor of military aid to Ukraine.
But fears have been expressed that the return of Donald Trump to the White House in January might slow, if not halt, the flow of American military aid to Kyiv.
The prime minister said the summit was “not just about sovereignty of Ukraine”, but also “our freedom, our democracy and our values”.
Following his talks with the Ukrainian leader, Sir Keir sidestepped a question about whether Trump’s presidential election victory was good for Europe and Ukraine.
He said he had met President Zelensky for the sixth time since becoming PM, adding it was an opportunity to affirm the UK’s “iron-clad support of Ukraine”.
Earlier, summit host Hungarian Prime Minister Viktor Orban said Europe’s leaders had agreed that they needed to take responsibility for their security and not just rely on the US for their defence.
“To be blunt, we cannot wait for the Americans to protect us,” Orban said.
The Hungarian leader is a staunch Trump supporter and has close ties to Moscow. He has been reluctant to impose sanctions on Russia or to supply Ukraine with weapons.
Trump has said he wants to end the Ukraine war “within a day”, but has declined to set out how this would be achieved.Some commentators have suggested it could mean the new US administration putting pressure on Zelensky to give up some territory as part of a peace deal with Russian President Vladimir Putin.
The Ukrainian leader said he had yet to discuss the conflict with the US president-elect.Sir Keir urged Ukraine’s allies to “step up” their backing, telling Zelensky: “As you know, our support for Ukraine is unwavering.”
It’s very important that we see this through. It’s very important that we stand with you.”The Ukrainian president thanked him “for sticking with us all through this tough period”.
French President Emmanuel Macron and German Chancellor Olaf Scholz have also repeatedly pledged to stand by Ukraine.
Nato Secretary General Mark Rutte said Trump’s first term had stimulated Europe to spend more on defence, but “we need to do more”.
He stressed that the threat of Russia, and its alliance with North Korea, China and Iran, posed problems for the US as well as Europe.
“If Russia would be successful in Ukraine, you would have an emboldened Russia at our border,” he said in Budapest.
Rutte, who was Dutch prime minister during Mr Trump’s first 2017-2021 presidency, added: “I worked with him very well for four years.
“He is extremely clear about what he wants. He understands that you have to deal with each other to come to joint positions. And I think we can do that.”
Sir Keir dodged a question about a report Trump had privately described him as “very left-wing”.
The prime minister said their meeting in New York in September and their phone call on Wednesday after the US election result had been “very positive, very constructive”.