Boris Johnson has told Rishi Sunak to "forget about Putin" and send fighter jets to Ukraine to help repel Russian invaders.
The ex-Prime Minister used a surprise trip to the US to condemn his old Cabinet colleague's decision not to arm Kyiv with warplanes - only hours after Downing Street said the idea was "not practical".
Asked about fighter jets, he told Fox News: "Save time, save money, save lives. Give the Ukrainians what they need as fast as possible. Get this thing done. Forget about Putin."
He added: "Every time we’ve said it would be a mistake to give such and such weaponry to Ukraine, we ended up doing it."
It marks Mr Johnson's latest intervention over the UK's support for Ukraine, which he regards as a key part of his legacy in Downing Street.
The former PM made an unannounced visit to Ukraine last month where he met President Volodymyr Zelensky and visited Bucha and Borodyanka to the north of Kyiv, the scenes of some of the worst atrocities committed by Russian forces.
Mr Johnson struck up a friendship with President Zelensky while in office, and attracted accusations that he was using calls to Kyiv to distract from his political troubles.
The scandal-hit Tory has found himself at the centre of a fresh storm after it emerged that his millionaire relative Sam Blyth had allegedly stepped in to act as a guarantor for up to £800,000 of credit to fund his lavish lifestyle in 2020.
New York-born Mr Johnson is set to demand more equipment for Kyiv when he addresses the Washington-based Atlantic Council think tank later today.
Downing Street rejected calls again on Wednesday to supply Ukraine with British fighter jets.
Asked about the former prime minister's trip to the US, Rishi Sunak's official spokesman said: "The Prime Minister welcomes all colleagues' backing for Ukraine and is pleased the former prime minister is continuing his staunch support of the United Kingdom's efforts to help Ukraine secure a lasting peace."
But he said that Mr Johnson is "acting in his own capacity and not on behalf of the UK Government".
On Tuesday, Downing Street said training Volodymyr Zelensky's forces on Typhoons and F-35s would take too long, but it does not oppose allies sending their own jets.
Defence Secretary Ben Wallace told the Cabinet that the US estimates that 180,000 Russian troops have been killed or injured in the year-long invasion.
That is compared to around 15,000 killed during the decade-long war in Afghanistan.
The Kremlin has also lost two-thirds of its tanks, Mr Wallace said, according to Downing Street's account of the meeting.
A national security official said Russian forces are suffering from equipment and munition shortages and have seen "significant attrition among some of their most capable fighting forces and officer corps".
Other Western officials were more cautious in their casualty estimates, suggesting both sides have lost more than 100,000 killed and wounded - with the Russians sustaining a higher proportion of fatalities.
"The Russians have been attacking and that means they have suffered more fatalities than the Ukrainians have," one official said.
The Prime Minister said his new strategy will be accompanied by "greater diplomatic efforts and planning work" with Ukraine "on how to rebuild" once the conflict is over.
But it does not include sending the advanced fighter jets Kyiv is demanding, which No 10 said was not discussed at Cabinet.
US President Joe Biden said on Monday he is not transferring warplanes to Kyiv despite the requests.
