British Prime Minister Keir Starmer said he would give Ukraine the support it needs to put it in the strongest position to fight Russia by 2025, Reuters reports.
Ahead of talks in Kiev with President Volodymyr Zelensky, he stressed: "We must not give up".
On his first visit to Ukraine since becoming prime minister in July last year, Starmer was keen to highlight Britain's support for the country just days before Donald Trump returns to power in the United States.
His visit comes two days after German Defense Minister Boris Pistorius also visited the country, as European leaders weigh what security guarantees could be provided as part of a possible peace deal that Trump could push through.
Trump's return to the White House has raised concerns that Moscow's attempt to end the war could force Ukraine to cede large swaths of its territory to Russia in the foreseeable future.
"It is very important that we ensure that Ukraine is in the strongest possible position," Starmer said after visiting patients at a hospital in Kiev that specializes in treating burns.
"That is why I had such intensive discussions with President Zelensky during the months I was prime minister and I will do so again here in Ukraine," he added.
"We are a long way from the end of this conflict. We must not give up.
As the war with Russia approaches its third year, Ukraine's position on the front line has weakened.
Ukrainian forces are suffering from a shortage of manpower and are losing ground in the Donetsk region as Russian troops continue their creeping advance there.
As Starmer met with officials in Kiev, a loud explosion was heard from Ukrainian air defense systems, which shot down Russian drones.
Since Russia's invasion in 2022, Britain has been a staunch supporter of Ukraine, with Starmer's predecessors visiting Kiev in the early days of their government.
It took Starmer a little longer to make the visit, but he comes "armed" with a 100-year partnership with Kiev to deepen security and cultural ties.
The treaty and political declaration aim to strengthen military cooperation to strengthen security in the Baltic, Black and Azov seas and deter Russian aggression.
The document will also cover areas such as energy, critical minerals and the production of green steel.
"Putin's ambition to tear Ukraine away from its closest partners was a monumental strategic failure," Starmer said. "Instead, we are closer than ever and this partnership will take that friendship to the next level.".
The UK has provided 12.8 billion pounds ($16 billion) in support of Ukraine since 2022. since - an amount that is less than Washington's $63.5 billion in security assistance, underscoring the importance of Trump's actions on Ukraine.
The partnership announced today, which provides £40 million for Ukraine's economic recovery, includes additional support around grain inspection and trade with Ukraine's booming technology sector, which has produced military equipment.