Boris Johnson has declared Donald Trump's diesel deal with Russia to be 'sickening'.
The once staunch supporter of the US President railed against the 'outrageous' agreement to lift sanctions on Kremlin fuel.
And he accused Mr Trump of forcing 'American drivers to bankroll Vladimir Putin's slaughter' to ease US pump prices, adding that the Royal Navy should intercept any Russian fuel ships in the English Channel.
His dramatic comments came as Mr Trump claimed an 'energy ceasefire' had been agreed between Russia and Ukraine.
The deal would mean both sides stop attacking each other's energy infrastructure, effectively helping Mr Trump's fuel agreement.
Speaking exclusively to the Daily Mail, the former prime minister said: 'The double standards are outrageous.
'Why should Putin be allowed to destroy Ukrainian energy infrastructure – as well as murdering countless civilians – if the Ukrainians are forbidden from responding?
'I doubt that Putin has the diesel to ship to the US but if he tries to go via the English Channel I hope it is intercepted by the Royal Navy.'
Boris Johnson has blasted Donald Trump's 'sickening' energy deal with Vladimir Putin
Mr Trump has agreed a deal to release Russian oil despite Moscow's continued attacks on Ukraine
CCTV images showed a blast from a Russian strike on Kharkiv, Ukraine, on Sunday
He added: 'The Ukrainians are valiantly trying to protect themselves and their families from threats of bombardment.
'We should be helping them and the best way to get this over and get the price of fuel down is to make Putin see sense.
'It's ridiculous to try to appease Putin and it won't make a bean of difference to the price of fuel.'
Mr Trump triggered a backlash after it emerged that he had agreed to buy nearly two million tonnes of Russian diesel over the next few months.
He is under pressure to reduce fuel prices as the US heads towards midterm elections next month, which are predicted to be a bloodbath for his Republican Party.
Ukrainian president Volodymyr Zelensky responded by describing the deal as a 'gift to Putin' that 'played into Russia's hands' and would extend the war. In response, Mr Trump said it was 'time for Ukraine to get a new president'.
He has blamed increasing fuel prices on Kyiv rather than his own war against Iran, which caused wider conflict in the Middle East.
The President warned that Ukraine 'better damn well stop' striking Russian oil infrastructure, adding: 'I suggest they get a new leader who can make a deal.'
Announcing the so-called ceasefire on Truth Social, he wrote: 'Effective immediately, there is an ENERGY CEASEFIRE in the War between Russia and Ukraine. Both parties have agreed. Do not break it!'
Mr Zelensky initially said the deal was 'news for him', but later said he supported the ceasefire and called for Russia to do the same.
Posting on X, he wrote: 'Ukraine supports the ceasefire, and it is important that Russia support it as well. We are awaiting details from the American side.
'Ukraine always FULLY supports steps toward de-escalation, the protection of lives and the decisions needed to end the war and guarantee lasting peace.'
He also said he was willing to organise elections in Ukraine if Russia stuck to a 60-day ceasefire, in an interview with US broadcaster ABC.
Volodymyr Zelensky, pictured in northern Ukraine with Foreign Secretary Ed Miliband this week, said he was unaware of Mr Trump's supposed 'energy ceasefire' but said he would agree to such a deal if Moscow did the same
Mr Zelensky said Russia was continuing its attacks on Ukraine and its energy infastructure as at least three died, including a child, in an attack on Kharkiv on Sunday
But on Sunday it appeared any ceasefire was in tatters with reports Russia had struck a power plant in Kyiv, followed by strikes on other energy facilities in the Kharkiv region.
Mr Zelensky said: 'And all of this happened AFTER the President... made his statement about an energy ceasefire.'
Mr Johnson – a staunch ally of Ukraine, who was the first Western leader to visit Kyiv after the Russian invasion – also rebuked Mr Trump on X, writing: 'It is sickening that the US is now lifting sanctions on Russian fuel, and transparently ridiculous to blame Ukraine for the spike in prices that obviously began with the closure of the Strait of Hormuz.
'Putin is murdering innocent Ukrainian civilians every day. The Ukrainians are fighting for their lives. And yet the White House is apparently determined to force American drivers to bankroll Putin's slaughter.
'It won't work and I doubt it will make much difference to the price of fuel.'
The intervention of the former Conservative leader will be a blow to Mr Trump given his previous support.
He was one of a handful of British politicians to attend the President's inauguration last year and said Mr Trump would provide 'strong leadership'.
But he has also shown unwavering loyalty to Mr Zelensky and Ukraine, and supported them on the international stage. Tory leader Kemi Badenoch backed Mr Johnson's position, saying Mr Trump was 'wrong' to strike the deal.
'Ukraine's freedom is not President Trump's to bargain away,' she said. 'He is wrong to ease sanctions on Russian diesel, wrong to blame President Zelensky for prolonging Putin's war and wrong to undermine a nation fighting for its right to exist.'
Defence Secretary Wes Streeting was less emphatic in his display of solidarity, telling Sky News: 'It's not the first time we've seen these sorts of tensions between Kyiv and Washington.
'Our role, our responsibility in this situation is to try to bring our friends back together.'
Mr Streeting suggested Mr Trump was grappling with the 'political reality' of domestic elections.
'Midterms are coming up,' he said. 'No doubt President Trump is feeling the same pressure that lots of leaders are facing in terms of wanting to ease cost-of-living pressures, energy prices.
'We would obviously prefer a stronger approach on sanctions.'
Russia struck Kharkiv's main power plant just hours after Mr Trump's 'energy ceasefire' announcement
Andy Burnham also called Mr Zelensky over the weekend to reaffirm Britain's support, following concerns that the US might halt intelligence sharing with Ukraine.
The Prime Minister reiterated his 'complete solidarity' with the Ukrainian leader and said he planned to meet him in person in the coming days.
Mr Trump has been urging Ukraine to stop strikes on Russian oil infrastructure so Russia can fulfil the deal to release diesel to the US. Kyiv, however, says the strikes are vital to limiting money funding Russia's war machine.
The deal means Moscow would export 300,000 tonnes of fuel immediately, followed by 500,000 tonnes in November and one million after that.
It comes just weeks after Mr Trump signed a sweeping sanctions bill, taking aim at Russia's economy by punishing major purchasers of its oil and gas.
Passed by the US House of Representatives last month, the Graham Act – named for Senator Lindsey Graham, who died earlier this year and was a staunch supporter of Ukraine – handed Mr Trump broad powers to levy tariffs of up to 100 per cent on the top five purchasers of Russian oil and gas, including China and India.
The President is under pressure to bring down fuel prices ahead of November's midterm elections that will decide control of Congress.
Diesel prices have rocketed from $3.68 per gallon a year ago to $6.28 in the US due to disruption to global supplies caused by the war against Iran, leaving the President facing an increasing backlash from US voters.
In his quest to reduce prices, Mr Trump has also threatened to restrict exports to G7 allies.