Ukrainian President Volodymyr Zelensky arrived in Britain on Monday to meet Prime Minister Andy Burnham, who used his first bilateral with a foreign leader to underline London's "unwavering support" for Kyiv.
The visit came ahead of Zelensky's trip to Washington on Tuesday, as he makes a new bid to end the four-year war with Russia.
Zelensky is expected to renew calls to boost defense as both Moscow and Kyiv intensify drone and missile strikes, with US-led negotiations stuck while Washington's attention has been diverted to the Iran war.
Zelensky and Burnham were to visit a naval base in the southern city of Portsmouth to speak to armed forces personnel involved in training Ukrainian service members.
Downing Street said Burnham would vow "to continue the unbreakable bond between the two nations", echoing what he told Zelensky in a call last Monday within hours of replacing Keir Starmer.
"Britain stands with Ukraine, shoulder to shoulder, and our support remains unwavering," Burnham said in a statement.
"Russia should be in no doubt of our resolve, and we will not back down until we achieve long-lasting and just peace for Ukraine."
Landing in Britain, Zelensky hailed the alliance as "the strongest relationship in the entire history" of the two countries.
London also announced it will share with Kyiv information on a new battle-tested electronic jamming capability dubbed "Stone Cloak".
When attached to drones, the tablet-seized, military-grade jammers can hinder Russian air defense systems from tracking and targeting such weapons in-flight, Downing Street said.
Thousands of the jammers, developed under a 100-year partnership pact between the countries, have already been gifted to Kyiv, according to London.
Following their roll-out and potential mass production in Ukraine, Stone Cloak will be built into the next generation of UK deep-strike capabilities, it noted.
"Stone Cloak is the best of homegrown British innovation and proven on the frontline, and it will be vital to protecting our security in both our countries," Burnham said.
In another sign of Ukraine's deepening defense ties with Britain, Zelensky told broadcaster Sky News on Sunday he wanted to sign a strong "drone deal" to boost production and technology-sharing.
He admitted "we don't have enough missiles to defend", referring to Russia's ultra-fast ballistic missiles that have put Ukraine's air-defense system under strain in a recent spate of deadly attacks.
Moscow "can increase the production of ballistic missiles and that's why we need air defense as quick as possible," Zelensky said.
Burnham and Zelensky will meet some of the approximately 200 Ukrainian military personnel who have been in Britain for the past three weeks participating in a maritime security and counter-mine exercise.
It was held to prepare for future missions in the Black Sea.
Downing Street noted the UK's total support for Ukraine has reached 25 billion pounds ($33 billion) since Russia's full-scale invasion in February 2022, including 16 billion pounds in direct military assistance.
Zelensky is due in Washington on Tuesday for talks with US President Donald Trump, a White House official said last week.
The Ukrainian leader is also expected to attend the funeral of US senator Lindsey Graham, a supporter of Ukraine who died earlier this month.
Trump and Zelensky have had a fraught relationship that has veered from a shouting match in the Oval Office in February 2025 to warmer ties in recent months.
The US president told Zelensky on the sidelines of a NATO summit this month that he was giving Ukraine permission to build US-designed Patriot air defense systems capable of downing ballistic missiles.
Washington has also advanced bipartisan legislation targeting countries buying Russian energy -- potentially clearing the way for stronger pressure on Moscow.
Trump's support for Ukrainian defense comes as the president faces domestic pressure to end the Middle East war.
"I understand the United States is focusing on the Middle East," Zelensky told Sky News.
"I try to manage it now with some Europeans and with Americans and with some other countries.
"This, for me, it's a big challenge, a big challenge for Ukraine."
