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.
About 2,000 people marched to the Russian embassy in west London ahead of the third anniversary of Vladimir Putin's full-scale invasion.
According to a Downing Street spokesperson the prime minister also said "Ukraine must be at the heart of any negotiations to end the war" and it could be sure of "the UK's commitment to securing a just and enduring peace to bring an end to Russia's illegal war".
Giving details about the phone call, Downing Street said Sir Keir and Zelensky "agreed that this was a significant moment for the future of Ukraine and European security at large".
Sir Keir also told Zelensky "that safeguarding Ukraine's sovereignty was essential to deter future aggression from Russia".
The prime minister added "he would be progressing these important discussions in the coming days and weeks, including with Trump whilst visiting Washington DC next week".
In a separate call on Saturday Sir Keir spoke with European Commission president Ursula von der Leyen and they agreed Europe "must step up for the good of collective European security", Downing Street said.
Sir Keir's meeting with Trump on Thursday comes after a week which saw a flurry of summits and phone calls as European leaders scrambled to work out how to approach the US president's sudden thawing of relations with Russia.
Ahead of visits to the White House by Sir Keir and French President Emmanuel Macron, Trump said the pair "haven't done anything" to end the war in Ukraine.
He also said Zelensky had "no cards" in peace negotiations and that he did not think "he's very important to be in meetings".
On Monday, European leaders held a hastily-arranged summit in Paris - a day before US-Russia talks in Saudi Arabia, and amid fears that Ukraine and Europe could be excluded from peace negotiations.
On Tuesday, Trump called Zelensky a "dictator" and said he should "never have started" the war, despite Russia invading Ukraine.
Zelensky responded by saying the US president was "living in a disinformation space" created by Russia.
On Saturday, people took to the streets of London in response to the US position on Ukraine, marching from outside the Ukrainian embassy to the Russian embassy.
Margaret Owen, 93, accused Trump of "appeasement", saying she remembered the Munich Agreement in which western powers signed a deal with Hitler in the years before the Second World War.
"It's outrageous. We can't let the world be dictated to by these two impossible people," she said of Trump and Putin.
The chair of the Foreign Affairs Select Committee, Emily Thornberry was among the marchers and argued for a less aggressive approach. The Labour MP said: "We want to influence the US president and we agree there must be peace. Why shout at him.
"You'd get a short term buzz from shouting at Trump but if you want to influence him, let's try and influence him."
"Ukraine needs to be at the table, you can't decide the future of Ukraine without Ukraine there and you can't just capitulate to Putin.
"They have to be invited into this process by the Americans and Russians."
Ukrainian Oleksandra Udovenko, who is from Kyiv but is studying in the UK, said: "I'm here to protect my country's interests, my country's independence, and my country's choice and my country's right to be independent of any empire in this world."
