British troops are lined up for an on-the-ground role in Ukraine once a ceasefire is reached.
France, Britain and Ukraine have signed a Declaration of Intent in Paris today on the future deployment of multinational forces as security guarantee for Ukraine in the event of a peace deal.
Announcing the measures, Sir Keir Starmer told a press conference in the French capital: "We signed a Declaration of Intent on the deployment of forces to Ukraine in the event of a peace deal.
"This is a vital part of our iron cast commitment to stand with Ukraine for the long term.
"It paves the way for the legal framework under which British, French and partner forces could operate on Ukrainian soil, securing Ukraine's skies and seas and regenerating Ukraine's armed forces for the future."
The Prime Minister said Britain and France will establish "military hubs" across Ukraine and build protected facilities for weapons and military equipment.
He added: "With our coalition partners, we have also agreed further steps.
"First, that we will participate in US-led monitoring and verification of any ceasefire. Second, we will support the long-time provision of armaments for Ukraine's defence.
Keir Starmer said the UK and France will set up 'military hubs' in Ukraine in the event of a ceasefire
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"And third, we will work toward binding commitments to support Ukraine in the case of a future armed attack by Russia."
Sir Keir went on to say that Russian leader Vladimir Putin is "not showing he is ready for peace".
"Over the last few weeks, we have seen the opposite," he said.
"Further horrific strikes on Ukraine, killing and wounding civilians and cutting off power for millions of people in the dead of winter.
"His forces hit a hospital in Kyiv juts yesterday and he has tried to distract from peace efforts with unfounded claims of attacks on his residence."
Leaders signed the Declaration of Intent in Paris today
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He warned the UK will continue to "keep the pressure up on Russia" including further measures on oil tankers and "shadow fleet operators funding Putin’s war chest".
The US special envoy to the Middle East, Steve Witkoff, said President Donald Trump, who brokered a peace deal between Russia and Ukraine last year, "wants peace" in Kyiv.
Speaking at the Paris press conference, he said: "President Trump's mandate is that he wants peace in Ukraine and we are determined on his behalf to do everything possible to attain that peace."
He added that "we think we have made a lot of progress" following the Coalition of the Willing meeting today.
According to a summary of the Ukraine peace deal released by the Elysee, the US is set to lead a “ceasefire monitoring and verification mechanism”, which the UK is expected to participate in.
A US guarantee, possibly in the form of air defence, has long been considered crucial to halting Russia from further military action in future.