Sir Keir Starmer has met with Ukrainian president at Downing Street as Europe braces for the outcome of Donald Trump’s face-to-face discussions with Vladimir Putin on Friday.
Sir Keir said Britain stands ready to “increase pressure” on Russia if necessary ahead of his meeting with Volodymyr Zelensky.
The Prime Minister warmly embraced Mr Zelensky on the steps of No 10 before the pair posed for photographs.
The pair agreed there was a "powerful sense of unity and a strong resolve" to secure peace in Ukraine during the meeting, Downing Street said.
In a readout of the meeting, a Number 10 spokesman said: "The Prime Minister hosted President Zelensky in Downing Street this morning.
"They had a private breakfast, where they discussed yesterday's meetings. They agreed there had been a powerful sense of unity and a strong resolve to achieve a just and lasting peace in Ukraine.
"They then looked ahead to tomorrow's talks between President Trump and President Putin in Alaska, which present a viable chance to make progress as long as Putin takes action to prove he is serious about peace.
"They agreed to stay in close touch in the coming days."
Meanwhile, Mr Trump threatened Russia with “severe consequences” if a ceasefire was rejected by its leader.
The US president and his Russian counterpart will meet in Alaska on Friday in what could be a decisive moment for the war in Ukraine.
During a call with Mr Trump and European allies on Wednesday, Sir Keir praised the US president for his work to bring forward a “viable” chance of an end to the war.
But concerns have been raised over Mr Zelensky’s exclusion from the meeting in Alaska, which critics have suggested gives the Russian president an opportunity to get in Mr Trump’s ear.
Speaking on Wednesday, Sir Keir said: “This meeting on Friday that President Trump is attending is hugely important.
“As I’ve said personally to President Trump for the three-and-a-bit years this conflict has been going on, we haven’t got anywhere near a prospect of actually a viable solution, a viable way of bringing it to a ceasefire.
“And now we do have that chance, because of the work of that the president has put in.”
Further sanctions could be imposed on Russia should the Kremlin fail to engage, and the UK is already working on its next package of measures targeting Moscow, he said.
“We’re ready to support this, including from the plans we’ve already drawn up to deploy a reassurance force once hostilities have ceased,” he told allies.
“It is important to remind colleagues that we do stand ready also to increase pressure on Russia, particularly the economy, with sanctions and wider measures as may be necessary.”
Sir Keir and European leaders have repeatedly said discussions about Ukraine should not happen without it, amid concerns the country is being sidelined in negotiations about its own future.
Asked if it was his decision to not invite Mr Zelensky to the meeting, Mr Trump said “no just the opposite”, before adding that a second meeting with the Ukrainian president could take place afterwards.
“We had a very good call, he was on the call, President Zelensky was on the call. I would rate it a 10, you know, very, very friendly,” he told reporters in Washington.
He added: “There’s a very good chance that we’re going to have a second meeting which will be more productive than the first, because the first is I’m going to find out where we are and what we’re doing.”
The US president has previously suggested a truce could involve some “swapping” of land.
It is believed one of the Russian leader’s demands is for Ukraine to cede parts of the Donbas region which it still controls.
But Mr Zelensky has already rejected any proposal that would compromise Ukraine’s territorial integrity, something that is forbidden by the country’s constitution.
A joint statement from the Coalition of the Willing, which is co-chaired by Sir Keir, French President Emmanuel Macron and German Chancellor Friedrich Merz, said “international borders must not be changed by force”.
It added: “Sanctions and wider economic measures to put pressure on Russia’s war economy should be strengthened if Russia does not agree to a ceasefire in Alaska.”
The Coalition of the Willing is a European-led effort to send a peacekeeping force to Ukraine in the event of truce.
