Ukraine has been told it has until Thursday to accept a humiliating peace deal or lose US military support.
Under terms thrashed out in secret between the White House and the Kremlin, Kyiv is expected to cede territory its troops have defended since February 2022, slash the size of its military and sacrifice its sovereignty.
Donald Trump has told his Ukrainian counterpart Volodymyr Zelensky that he has until Thursday to make up his mind.
The 28-point peace plan paves the way for the United States to pocket a multi-billion-dollar payout, including compensation for supporting Ukraine militarily and deals with Russia for rare earth minerals.
In a dramatic video address, Mr Zelensky said: 'Today is one of the most difficult moments in our history.
'The pressure on Ukraine is now at its most intense. Ukraine may now face a very difficult choice: either the loss of dignity, or the risk of losing a major security partner – 28 points or an extremely harsh winter.
'Ukraine's national interest must be taken into account. We're not making loud statements; we'll be calmly working with America and all the partners. We'll have a constructive search for solutions with our main partner [the US].'
Mr Zelensky said he would work around the clock to find a path, but insisted he would not betray his country.
Last night, former defence secretary Ben Wallace launched a stinging attack on the US President.
He said: 'When will European leaders wake up and recognise that Donald Trump is only interested in dollars and running a protection racket? He has never had any intention of supporting Ukraine. Time for all of us to invest in our defence and to stand together for Ukraine.'
Predictably, Russian president Vladimir Putin welcomed the plan, saying it could 'form the basis of a final peace settlement'.
Putin also accused Ukraine and its allies of 'dreaming' that Russia can be defeated militarily.
Last night a government source described the situation as 'on a knife-edge'. Sir Keir Starmer pledged his support to Ukraine, but there was no criticism of President Trump.
Speaking at the G20 summit in South Africa, the Prime Minister said: 'Ukraine has been ready to negotiate for months while Russia has stalled and continued its murderous rampage. That is why we must all work together with both the US and Ukraine to secure a just and lasting peace once and for all.'
Former head of the military think-tank RUSI, Professor Michael Clarke, described the situation as 'the most critical moment of the collapse of the transatlantic relationship'.
Behind the scenes, the UK has launched a diplomatic offence, alongside allies such as France and Germany, to suggest counter-proposals and convince the US to soften its stance.
But the reluctance of the White House to listen to alternative arguments was signalled by Sir Keir's failure to speak to Mr Trump. The radio silence came despite the UK playing its strongest diplomatic cards, including King Charles's personal request for the US President to receive a second state visit.
Despite repeatedly extending deadlines set for Russia to agree a ceasefire or enter negotiations, President Trump confirmed last night: 'Thursday is it.'
Ukraine faces being cut off militarily by the US unless it agrees terms, with weapon supplies drying up and an intelligence-sharing agreement coming to an end.
With European support, Ukraine could continue to withstand Russia's aerial bombardment indefinitely, but at an increasing cost in terms of lives lost and territory conceded.
Standing in the rain outside a government building, Mr Zelensky said: 'I will present arguments, I will persuade, I will offer alternatives, but we will definitely not give the enemy any reason to say that Ukraine does not want peace, that it is disrupting the process and that Ukraine is not ready for diplomacy.'
He also recalled how Ukraine responded in the immediate aftermath of Russia's invasion in February 2022: 'We did not betray Ukraine then, we will not do so now.'
Mr Zelensky is expected to hold a crisis phone call with Mr Trump early next week. He has already spoken to Sir Keir and French president Emmanual Macron.
Downing Street officials considered whether Sir Keir should return to Britain, but instead, European leaders are expected to hold a meeting on the sidelines of the summit in Johannesburg today to discuss the crisis.
