Boris Johnson has issued a rallying cry against the “cruel and criminal” war waged by Putin’s Russia against Ukraine. As European leaders gathered in Kyiv, just days after marking 80 years since Victory in Europe Day, the former Tory leader likened the Russian leader to Adolf Hitler, and urged Britain and its allies to seize Russia’s $300billion (£225bn) in frozen assets and back a new round of sanctions.

Pressing Brits to stand strong with a “heroic” Ukraine as Volodymyr Zelensky's nation resists the invasion, he warned that Putin “will not stop the violence until he has no alternative”.

Mr Johnson told the Sunday Express: “We need finally to put the squeeze on Putin and we need to show him once and for all that he will never conquer Ukraine.”

The former Prime Minister’s intervention comes at a crucial stage in the war, with Sir Keir Starmer yesterday joining fellow European leaders in Kyiv to push Russia to agree to a 30-day ceasefire. He travelled by train with French President Emmanuel Macron and newly appointed German Chancellor Friedrich Merz.

Sir Keir warned that if Putin “turns his back on peace” then Russia will face ramped up sanctions and Ukraine will receive increased military aid.

The Prime Minister told the Sunday Express the “battle lines of Ukraine are the frontlines for our values and our freedoms”.

He said: “We will not hesitate to defend our democracy, stand tall alongside our allies, and show the world, once again, that Britain is a country that will always stand up to tyranny.”

Sir Keir and his Ukrainian, French, German and Polish counterparts spoke to US President Donald Trump to update them on the joint push for a breakthrough. Mr Trump has promised “further sanctions” from the US and “partners” if any ceasefire is struck and then not respected.

The PM took inspiration from the VE Day celebrations, telling Sunday Express readers: “It’s so important to celebrate and remember VE Day because we didn’t just win victory for Britain in 1945. We won a battle against the forces of tyranny, oppression and hatred.

“I met brave veterans this week who told me how much their generation sacrificed. The best way we can honour their memory today is to stand firm again against anyone that seeks to threaten the peace that was hard fought and hard won.”

Former Tory leader Mr Johnson pushed for a long-term commitment to Ukraine’s security, saying: “We need to show that we will sincerely back President Trump’s peace plan with British and other European forces. We need a 10-year plan to supply the Ukrainians with weapons from the West to make that country invulnerable to further attack.

“That is the only way to bring peace to Ukraine.”

Mr Zelensky said the leaders’ conversation with the US President had been “positive and concrete”.

He said: “I am grateful to President Trump. We share a common view: an immediate, full and unconditional ceasefire is needed for at least 30 days.

“We propose it begin on Monday, May 12. We are waiting for Russia’s response.”

The gathering of European leaders in Kyiv contrasted with the VE Day commemorations in Moscow. There, the Russian autocrat was joined by China’s Xi Jinping and leaders from Brazil, Venezuela, Serbia and Slovakia.

Sir Keir said “all of us here, together with US, are calling Putin out”.

Justin Crump of the Sibylline strategic risk group said the visit by the European statesmen was significant.

“I think the timing for this visit, on the day after Putin showed off a collection of world leaders at his Victory Day parade, is notable,” he said.

“There are many reasons to show a strong force in Ukraine, and there have been large delegations visiting before – though this is the first that Starmer has led, and it shows our role in building the alliance.

“Britain is leading the donor group now, equipping the Ukrainians, and is driving their special forces operations. We are even targeting for them.”

He called reports that Starmer had managed to persuade Trump to have more sympathy with Ukraine’s case “believable”.

“I don’t know whether Trump has completely changed his mind on Russia and Ukraine, but there has been progress,” he said. “It’s going to require persistence and long-term effort and engagement to shift his mind – and keep it there.

“The British and the French strategy with Trump has been to flatter, engage, gradually persuade and cajole. They’ve been working steadily to point out that the Russians aren’t really serious about peace, that they could stop if they wanted to, and that Trump has given them chances.

“And whatever Starmer’s faults may or may not be, he’s a very good barrister – good at presenting arguments, seeing other points of view, and using it. That’s what his skill set is beyond politics.”