President Volodymyr Zelenskiy was due in London on Wednesday for only his second trip abroad since Russia invaded Ukraine last year, seeking additional arms to combat an expected major assault.
British Prime Minister Rishi Sunak's office said Zelenskiy would visit troops training in Britain and address the British parliament.
Sunak's office announced plans to expand training for Ukraine's military to include fighter jet pilots and marines - and accelerate the supply of military equipment.
"President Zelenskiy's visit to the UK is a testament to his country's courage, determination and fight, and a testament to the unbreakable friendship between our two countries," Sunak said in the statement.
Zelenskiy visited the United States in December and addressed the U.S. Congress on his only trip abroad since Russia launched its full-scale invasion in February last year.
Western countries have dramatically scaled up their pledges of military support for Ukraine since the start of the new year, including promises of hundreds of tanks and armoured vehicles. Kyiv still wants longer range missiles and war planes.
After major Ukrainian gains in the second half of 2022, Russia has recovered momentum, with tens of thousands of freshly mobilised troops reaching the front.
continues, ahead of EU summit in Kyiv, Ukraine February 2, 2023. Ukrainian Presidential Press Service/Handout via REUTERS
Russian forces have made incremental progress in relentless winter battles in the east of the country in recent weeks which both sides describe as some of the bloodiest fighting of the war.
Kyiv says it expects Moscow to broaden that offensive with a big push as the Feb. 24 first anniversary of the invasion approaches.
"They need to have something to show before their people, and have a major desire to do something big, as they see it, by this date," Ukraine's national security chief Oleksiy Danilov told Reuters on Tuesday in an interview.
He predicted Russia, which has focused lately on the Donetsk region in the east, would try new attacks on Kharkiv further north or Zaporizhzhia further south.
"How successful they'll be will depend on us."
Russia launched its "special military operation" to combat what it describes as a security threat from Ukraine's ties to the West, and says Western supplies of weapons to Kyiv will only prolong the war. It claims to have annexed four Ukrainian provinces last year.
Ukraine says the only way to stop the war is for the West to give it the capability to drive Russian forces out.