Keir Starmer and other European leaders have warned Vladimir Putin he faces “massive” new sanctions and increased military aid to Ukraine if he does not agree to an almost immediate 30-day ceasefire.
The prime minister said they were “calling out” the Russian leader with the backing of Donald Trump. “If [Putin] is serious about peace, he has a chance to show it now,” Sir Keir said.
And he warned there were “no more ifs and buts” after he flew to Kyiv to stand side by side with President Zelensky, saying Putin had not required extra conditions to be met when “he wanted a ceasefire to have a parade – and he does not need them now”.
In an extraordinary turnaround just weeks after President Trump’s showdown with Zelensky in the Oval Office, Sir Keir also said Europe and the US were “speaking with one voice” on the issue.

Trump has also threatened extra sanctions if a 30-day truce, which has already been agreed by Ukraine, fails to start on Monday.
Sir Keir also said Putin would face countries increasing “our military aid for Ukraine’s defence to pressure Russia back to the table” if he did not act.
Before a joint press conference, Sir Keir, French President Emmanuel Macron, German Chancellor Friedrich Merz and Polish Prime Minister Donald Tusk spoke with Trump about their plans.
Unplanned, the call is understood to have lasted around 20 minutes and was described as “warm” by a UK source.
The European leaders said the terms of a peace deal would be negotiated during the 30-day pause in fighting.
However, Dmitry Medvedev, a former Russian president who is often Putin’s attack dog when it comes to criticising the West, derided the ultimatum.
“Shove these peace plans up your pangender a***s!” he wrote on X.

Sir Keir said the sanctions threat was the result of the “greatest unity we have ever had” among other countries. Europe and the US were “all speaking with one voice”, he added.
UK and European experts were working to “boost Ukraine’s economy”, including the resumption of flights into the country, when peace is achieved, he added.
It is the first time the leaders of the four countries have travelled together to Ukraine, and marks Mr Merz’s first visit since he was elected chancellor on Tuesday. Before their trip, Sir Keir and the other leaders called on Russia to “stop obstructing efforts” to secure a peace deal.
The show of strength comes after the US president called for “ideally” a 30-day ceasefire, and warned that if any pause in the fighting was not respected, “the US and its partners will impose further sanctions”.
In the Ukrainian capital, the European leaders joined Mr Zelensky and first lady Olena Zelenska to place candles at a makeshift memorial to killed Ukrainian soldiers in the city’s Independence Square during a ceremony marking the 80th anniversary of the end of the Second World War in Europe.
On Friday, Lord Peter Mandelson, the UK’s ambassador to Washington, said that the UK had “influenced” Mr Trump’s views on Ukraine.
He told the BBC’s Newsnight: “We have influenced his outlook on Ukraine in our attitude to Putin.
“You’ve seen how his stance and his language has changed just in recent weeks and indeed recent days.”