Sir Keir Starmer will join other European leaders in Kyiv on Saturday for talks on the "coalition of the willing".
The prime minister is attending the event alongside French President Emmanuel Macron, recently-elected German Chancellor Friedrich Merz and Polish Prime Minister Donald Tusk.
It will be the first time the leaders of the four countries will travel to Ukraine at the same time - with their meeting hosted by President Volodymyr Zelenskyy.
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Military officers from around 30 countries have been involved in drawing up plans for the coalition, which would provide a peacekeeping force in the event of a ceasefire being agreed between Russia and Ukraine.
Ahead of the meeting on Saturday, Sir Keir, Mr Macron, Mr Tusk and Mr Merz released a joint statement voicing support for Ukraine and calling on Russia to agree to a 30-day ceasefire.
"We reiterate our backing for President Trump's calls for a peace deal and call on Russia to stop obstructing efforts to secure an enduring peace," they said.
"Alongside the US, we call on Russia to agree a full and unconditional 30-day ceasefire to create the space for talks on a just and lasting peace."
The leaders said they were "ready to support peace talks as soon as possible".
But they warned that they would continue to "ratchet up pressure on Russia's war machine" until Moscow agrees to a lasting ceasefire.
"We are clear the bloodshed must end, Russia must stop its illegal invasion, and Ukraine must be able to prosper as a safe, secure and sovereign nation within its internationally recognised borders for generations to come," their statement added.
"We will continue to increase our support for Ukraine."
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The European leaders are set to visit the Maidan, a central square in Ukraine's capital where flags represent those who died in the war.
They are also expected to host a virtual meeting for other leaders in the "coalition of the willing" to update them on progress towards a peacekeeping force.
This force "would help regenerate Ukraine's armed forces after any peace deal and strengthen confidence in any future peace", according to Number 10.
(c) Sky News 2025: European leaders to meet in Ukraine for 'coalition of the willing' talks - and issue call to R
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