Sir Keir Starmer has arrived in Kyiv to sign what Downing Street is calling a “landmark 100-year partnership” with Ukraine.
The pact would formalise economic and military support already pledged to the country, and offers more.
It is the prime minister’s first visit to the country since taking office last summer, in a show of support for Ukraine days before Donald Trump re-enters the White House.
Ukrainian President Volodymyr Zelensky is keen to discuss firm security guarantees from key allies such as the UK, wary that a new US administration could start pushing Ukraine to make peace with Russia.