Boris Johnson will travel to Ukraine on Tuesday for talks with the country’s president, amid rising concerns over a possible Russian invasion.
The UK prime minister promised to work with Volodymyr Zelensky to find a diplomatic solution to arguments with Moscow and “avoid further bloodshed”.
Russia has massed tens of thousands of troops near its border with Ukraine in recent weeks.
But President Vladimir Putin denies he is planning an attack.
However, his government has warned Ukraine not to make any further moves towards joining Nato, arguing that this threatens Russia’s own security.
The US has rejected this demand and Western countries are threatening to expand sanctions against Moscow if there is an invasion.
Ahead of Mr Johnson’s visit to Ukraine’s capital, Kyiv, the UK government announced it was giving £88m to promote stable governance and energy independence from Russia.
Mr Johnson said: “It is the right of every Ukrainian to determine how they are governed. As a friend and a democratic partner, the UK will continue to uphold Ukraine’s sovereignty in the face of those who seek to destroy it.
“We urge Russia to step back and engage in dialogue to find a diplomatic resolution and avoid further bloodshed.”
Mr Johnson hopes also to speak to Mr Putin later this week.
Foreign Secretary Liz Truss had been due accompany the prime minister to Ukraine, but she announced on Monday that she had been diagnosed with Covid and was self-isolating.
US President Joe Biden has warned there is a “distinct possibility” of Russia invading Ukraine within the next month.
Moscow’s forces annexed Ukraine’s southern Crimea peninsula in 2014 and it is backing rebels who seized large swathes of the eastern Donbas region soon afterwards.
Some 14,000 people have died in fighting there.