Keir Starmer has warned Russia does "not hold all the cards" in peace talks with its economy reeling from its three-year long invasion of Ukraine.
The PM also claimed sanctions - set to be announced by Foreign Secretary David Lammy later today - will force Vladimir Putin not only to talk "but to make concessions".
He said they will be the "largest package of sanctions since the early days of the war" - with some targeting Russia's shadow fleet and firms in China sending military components.
Mr Starmer's words came as he joined a call with Ukraine's war time leader Zelensky and European leaders to mark the third anniversary of Moscow's full-scale invasion. Mr Starmer, who praised Zelensky's leadership, also revealed the message a patient named Petro delivered to him on a burns unit during a recent visit to Kyiv.
"He said to me 'if Ukraine fails, Europe will be next'. And that is what is at stake here," the PM said. "That is why we will always stand with Ukraine and our allies against this aggression and for a just and lasting peace".
But the PM added: "This is a time for unity. In this crucial moment, as talks begin, we must work together to shape the outcome. Russia does not hold all the cards in this war, because the Ukrainians have the courage to defend their country, because Russia’s economy is in trouble, and because they have now lost the best of their land forces and their Black Sea Fleet in this pointless invasion."
Be the first with news from Mirror Politics
BLUESKY: Follow our Mirror Politics account on Bluesky here. And follow our Mirror Politics team here - Lizzy Buchan, Mikey Smith, Kevin Maguire, Sophie Huskisson, Dave Burke and Ashley Cowburn.
POLITICS WHATSAPP: Be first to get the biggest bombshells and breaking news by joining our Politics WhatsApp group here. We also treat our community members to special offers, promotions, and adverts from us and our partners. If you want to leave our community, you can check out any time you like. If you’re curious, you can read our Privacy Notice.
NEWSLETTER: Or sign up here to the Mirror's Politics newsletter for all the best exclusives and opinions straight to your inbox.
PODCAST: And listen to our exciting new political podcast The Division Bell, hosted by the Mirror and the Express every Thursday.
Later this week, Mr Starmer will jet to Washington for his first face-to-face talks with Donald Trump since the Republican leader returned to the Oval Office. The PM told European leaders and Zelensky that Mr Trump had "changed the global conversation" around the ongoing war.
He said: "It has created an opportunity. Now we must get the fundamentals right. If we want peace to endure, Ukraine must have a seat at the table, and any settlement must be based on a sovereign Ukraine backed up with strong security guarantees.
"The UK is ready and willing to support this with troops on the ground, with other Europeans and with the right conditions in place, and ultimately, a US backstop will be vital to deter Russia from launching another invasion in just a few years' time. So we will do everything we can to get the best outcome for Ukraine and for us all."
But Mr Starmer also expressed solidarity with Kyiv after a war of words erupted between the US and UK. In the last few days Mr Trump has attacked Ukraine's president and blindsided European leaders by excluding Kyiv from negotiations over peace talks with Russia.
The PM said: I have a very simple, clear message today: the UK is with you. Today and every day. From His Majesty the King to the NHS workers volunteering in hospitals in Ukraine to the communities that took Ukrainian refugees to their heart.
"And that's why I signed our 100-year partnership with President Zelenskyy last month because we believe in Ukraine's fight today, and the country's incredible potential to thrive in the years to come. "