Sir Keir Starmer travels to Kiev to sign what Downing Street calls a “historic 100-year partnership” with Ukraine.
The agreement would formalize economic and military support already pledged to the country, and provide more.
This 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 returns to the White House.
Ukrainian President Volodymyr Zelensky is keen to discuss tough security guarantees from key allies such as the UK, and fears that the new US administration will begin to push Ukraine to make peace with Russia.
Unlike other prime ministers who rushed to visit Kiev, Sir Keir took his time to visit, but after six months in office he is now coming to Ukraine pledging long-term support against what he calls Russia's “illegal and barbaric invasion”.
“It's not just about the present, it's also about investing in our two countries for the next century,” he said while traveling to the country.
He added, “(Russian President Vladimir) Putin's ambition to wrest Ukraine from its closest partner was a massive strategic failure. Instead, we are closer than ever, and this partnership will take that friendship to the next level.”
Zelensky is keen to discuss tough security guarantees from key allies, fearing that the new US administration under President-elect Donald Trump will begin to push Ukraine to make peace with Moscow.
Marco Rubio, Trump's pick for secretary of state, said earlier this week that both countries would have to make concessions to see an end to the war.
Thursday's announcement includes more military and economic aid to demonstrate this, as well as increased military cooperation in maritime security, drone technology and health care.
Zelensky previously said he looked forward to help from the UK for security guarantees to deter future attacks.
Joining NATO tops his wish list, but Ukraine also wants its allies to send peacekeepers into the country if the fighting stops, to patrol the current frontline that could become a buffer zone in any peace deal.
Before the visit, Zelensky said that this was something he would discuss with the prime minister.
This builds on the £12.8bn of support the UK has already provided to Ukraine. The country has already committed to giving the country £3bn in military aid every year “for as long as it takes”.
Ukraine is already using British-supplied Storm Shadow missiles to attack Russian military installations far from the border.
Their arrival was welcomed by Kyiv late last year, and Moscow criticized it.
The partnership, which consists of a treaty and a political declaration, is due to be submitted to Parliament in the coming weeks.
Plans for this began under the previous Conservative government.
Starmer previously visited Ukraine when he was opposition leader in 2023, and has hosted President Zelensky twice in Downing Street since taking office.
Additional reporting by Alex Smith.
