
Sir Keir Starmer arrived in Kiev to sign what Downing Street called a “remarkable 100-year partnership” with Ukraine.
The pact would formalize the economic and military support already promised to the country and offer 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, fearing that a new US administration could begin pressuring Ukraine to make peace with Russia.
Unlike other prime ministers who rushed to Kiev, Sir Keir took time out to visit, but after six months in office he came to Ukraine pledging long-term support against what he called Russia's “illegal and barbaric invasion”.
He was met at the railway station in Kyiv by the UK ambassador to Ukraine, Martin Harris, and the Ukrainian envoy to London, Valery Zaluzhny.
The highlight of the visit will be the signing of the 100-year partnership agreement, Zuluzhni said, which will cover areas from science to culture, with a special emphasis on security.
Speaking on his trip to the country, Sir Keir said: “This is not just here and now, but an investment in our two countries for the next century.”
“(Russian President Vladimir) Putin's ambition to cut off Ukraine from its closest partners was a monumental strategic failure. Instead, we are closer than ever and this partnership will take this friendship to the next level,” he added.
Trump's pick to become secretary of state, Marco Rubio, said earlier this week that both sides would have to make concessions to see the war end.
Thursday's announcement includes more military and economic aid to prove it, as well as increased military cooperation on maritime security and drone technology, as well as health care.
Zelensky previously said he was seeking help from the UK to obtain security guarantees to deter future attacks.

Joining NATO is near the top of his wish list, but Ukraine also wants its allies to send peacekeepers into the country if fighting stops to patrol the current front line, which could become a buffer zone in any peace deal.
Before the visit, Zelensky said it was something he would discuss with the prime minister.
It builds on £12.8bn of support the UK has already provided to Ukraine. The country has also already committed to providing the country with £3bn of military aid each year “as necessary”.
Ukraine already uses British-supplied Storm Shadow missiles to attack Russian military installations far from the border.
Their arrival late last year was welcomed by Kiev and criticized by Moscow.
The partnership, made up of a treaty and a political declaration, is due to be presented to parliament in the coming weeks.
Plans for this were started under the previous Conservative government.
Sir Keir previously visited Ukraine when he was opposition leader in 2023 and has hosted President Zelensky twice at Downing Street since taking office.
Additional reporting by Alex Smith.
