Sir Keir Starmer and Volodymyr Zelensky have agreed on the "importance of refining" the UK's training of Ukrainian forces to "bolster" the country's defence.
In a call with the Ukrainian president on Monday, the prime minister said Western allies were "united in their determination to put Ukraine in the strongest possible position going into 2025".
It comes after it emerged the Ministry of Defence was considering the possibility of sending British troops to Ukraine to help train its armed forces.
Britain has been training Ukrainian troops in the UK since 2022, while there is also a small team of British Army medics providing training inside Ukraine.