Sir Keir Starmer spoke to Donald Trump ahead of the US president’s crunch talks with Russia’s Vladimir Putin.
The Prime Minister told Mr Trump that Ukraine must be put in the “strongest possible position” in order to secure a “just and lasting peace” in the war with Russia.
Mr Trump and his Russian counterpart are due to speak on Tuesday about the US-Ukraine plan for a 30-day ceasefire.
Following a call between Sir Keir and Mr Trump on Monday night, the Prime Minister’s official spokesman said: “The Prime Minister updated the president on his coalition of the willing call with international leaders that took place on Saturday.
“He reiterated that all must work together to put Ukraine in the strongest possible position to secure a just and lasting peace.”