Sir Keir Starmer is facing pressure to be bold with Donald Trump when he travels to Washington next week, amid a slew of unproven claims about Ukraine that have shaken Western political relations.
The US president has traded barbs with his Ukrainian counterpart Volodymyr Zelensky over the past week, blaming Kyiv for starting the war and claiming Russia holds all the cards in peace talks.
Mr Zelensky was also branded a “dictator” by Mr Trump, something Sir Keir and other UK political figures have pushed back against.
The Ukrainian president claimed Mr Trump was living in a Russian “disinformation space” as he hit back, which led White House officials to accuse Mr Zelensky of “insulting” his counterpart.
The Americans also cancelled a planned joint press conference in Kyiv, in a sign of a deepening feud between the two countries.
Businessman Elon Musk, who is acting as an adviser on federal spending to Mr Trump, meanwhile suggested Mr Zelensky is running a “fraud machine feeding off the dead bodies of soldiers”.
Mr Musk was throwing his support behind the US president’s contested claim that Mr Zelensky’s polling popularity has fallen dramatically.
The Prime Minister will travel to America next week, to meet face-to-face with Mr Trump for the first time since he became president.
US support for Ukraine, including in enforcing peacekeeping efforts should a ceasefire be brokered, will likely be on the agenda for the meeting.
But calls are growing for Sir Keir to take a hard line with the US leader, and challenge him on his claims about Ukraine and US-Russian talks on an end to the war.
Sir Ed Davey, the Liberal Democrat leader, urged the Prime Minister to speak “honestly and openly” with Mr Trump.
He added: “The threat that Donald Trump poses to our economy and security is as serious as I can ever remember.
“Whether it’s the threat of terror to our country, indeed to our friends in the Commonwealth and Europe, or whether it’s what he’s saying with President Putin and Russia and Ukraine, I think we’re all astonished and deeply alarmed, and if the British Prime Minister doesn’t reflect that, he’s not reflecting the views of the British people.”
The Lib Dem leader also urged Sir Keir to swiftly meet his commitment to raise defence spending to 2.5% of gross domestic product (GDP), a measure of national economic output.
He suggested raising digital services tax on multinational companies from 2% to 10% would pay for the hike in defence spending.
European nations met earlier this week in a scramble to boost their defence spending after pressure from the US.
American and Russian officials, meanwhile, met in Saudi Arabia for initial talks on ending the conflict, without any Ukrainian representation.
But speaking after a G20 meeting of foreign ministers in Johannesburg, South Africa, on Thursday, Foreign Secretary David Lammy cast doubt on Moscow’s willingness to negotiate seriously.
He said: “We’ve not got anywhere near a negotiated settlement, and I have to say when I listened to what the Russians and what (Russian foreign minister Sergei) Lavrov has just said in the chamber this afternoon, I don’t see an appetite to really get to that peace.”
Both Mr Lammy and Mr Lavrov gave speeches in a behind-closed-doors session in Johannesburg.
The text of Mr Lavrov’s speech has not been released and it is understood the Russian foreign minister boycotted Mr Lammy’s speech.
According to a copy of Mr Lammy’s remarks posted on the Foreign Office’s website, he accused his Russian counterpart of advancing “tired fabrications” and urged Moscow to abandon “tsarist imperialism”.
The meeting of G20 foreign ministers is to continue on Friday, and Mr Lammy is expected to also hold bilateral meetings with his counterparts from South Africa, Saudi Arabia and Singapore.
