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Sir Keir Starmer will discuss the importance of Ukraine's sovereignty in talks with Donald Trump next week, he said in a call with the country's president.
The UK prime minister reiterated the UK's "ironclad support" for Kyiv when he spoke to Volodymyr Zelensky on Saturday.
The two leaders held their second telephone conversation in four days following US President Trump's decision to re-open relations with Russia and seek an end to the war in Ukraine.
Monday marks three years since Vladimir Putin's invasion, which UK Foreign Secretary David Lammy said he will mark with further sanctions on Russia.