Sir Keir Starmer is pushing Western allies to step up support to put Ukraine in the “strongest position” possible before Donald Trump takes office.
The return of Mr Trump to the White House means there is uncertainty over the level of US support for Kyiv from January when the president-elect takes office.
The Prime Minister said he had been having discussions with other leaders about how to shore up support for Volodymyr Zelensky’s forces.
Sir Keir held talks with France’s Emmanuel Macron on Monday and is reported to be pushing for outgoing US President Joe Biden to release £15.6 billion in funds for Ukraine before he leaves office.
The Prime Minister hopes to talk face-to-face with the president at the G20 summit next week, The Daily Telegraph reported.
The Biden White House could also be pushed to give permission for Ukraine to use UK-supplied Storm Shadow missiles to strike targets within Russia.
Concerns about Mr Trump’s stance on Ukraine have been heightened by a New York Times report that Marco Rubio is being lined up as secretary of state.
Mr Rubio was one of the senators who voted against a US funding package for Kyiv earlier this year.
Sir Keir insisted he did not want to tell Mr Trump how to act, but stressed the UK’s unwavering support for Ukraine in its fight against Vladimir Putin’s Russia.
Asked what his message to Mr Trump was, the Prime Minister told reporters at the Cop29 climate summit in Azerbaijan: “I’m not going to start sending messages to the president-elect but I’m very happy to be very clear about what my own position is, because it’s been my position since the very start, which is that we strongly support Ukraine in the face of Russian aggression.
“I’m very proud of the fact that the UK has stood firm in that commitment throughout the duration of this conflict.
“I now think that we need to put Ukraine in the best possible position as we go forward.
“That has been the centre of the discussions that I’ve been having with other leaders in recent weeks.
“So, strong support for Ukraine, we want to put Ukraine in the best possible, strongest position as we go forward.
“And I’m not going to be drawn into discussions about capabilities as we go forward but that is the strong and in principle position that I take.”
Nato secretary general Mark Rutte, who was holding his own talks with Mr Macron on Tuesday, stressed the need to back Ukraine for the “long haul”.
He highlighted the “importance of keeping up the support as Ukrainians prepare to face what could be their harshest winter since 2022”.
Mr Rutte added: “We must recommit to stay in the course for the long haul. And we must do more than just keep Ukraine in the fight.
“We need to raise the cost for Putin and his enabling authoritarian friends, by providing Ukraine with the support it needs to change the trajectory of the conflict.”
