Boris Johnson has piled pressure on Western leaders to supply fighter jets to Ukraine, setting up a clash with Rishi Sunak.
The UK said it is not practical to provide sophisticated planes used by the RAF, such as Typhoons and F-35s, and US President Joe Biden has ruled out sending F-16 fighters.
Downing Street stressed that a visit by Mr Johnson to Washington to bolster support for Ukraine was “not on behalf of the UK Government”.
The former prime minister, who held talks with senior politicians to lobby for Ukraine, said the country’s president, Volodymyr Zelensky, should be given all the equipment he needs.
Asked about the situation, Mr Johnson told Fox News: “All I will say is that every time we have said it will be a mistake to give such and such an item of weaponry, we end up doing it and it ends up being the right thing for Ukraine.
“I remember being told it was the wrong idea to give them the anti-tank shoulder-launched missiles. Actually, they were indispensable and the United States – under (former US president) Donald Trump – gave them the Javelins as well.
“They were indispensable in the battles to repel the Russian tanks.
“People said that we shouldn’t give the Himars (rocket launchers). I remember having arguments about the multiple-launch rocket systems, the MLRS.
“Actually, they have proved invaluable to the Ukrainians.
“We said the same about tanks.
“All I’m saying is save time, save money, save lives. Give the Ukrainians what they need as fast as possible.”
Mr 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 Mr Johnson is “acting in his own capacity and not on behalf of the UK Government”.
Asked about Mr Johnson’s jets call, the official said: “It’s currently not practical to send UK jets. We will continue to work closely with the Ukrainians to understand their needs and how allies can further support them.
“Given the complexity of UK fighter jets and the length of time required to train them we do not currently think it is practical to do so.”
Downing Street said the RAF’s Typhoons and F-35s are “extremely sophisticated” pieces of equipment.
A Royal Air Force chief suggested the UK’s stance on providing Ukraine with jets could change.
Giving evidence to the Commons Defence Committee on Wednesday, Air Chief Marshal Sir Mike Wigston said: “I think the Prime Minister and the Secretary of State yesterday were very clear on where we stand on fast jets at this stage.
“But these are things that, the course of the invasion – of the brutal invasion and Ukraine’s heroic defence and how we have supported them – that has been an evolutionary process and I look to the future and we should be ready for any outcomes.”
He declined to say what air power the RAF could make available to Ukraine.
Mr Biden said on Monday that he will not send warplanes to Kyiv.
Meanwhile, Mr Johnson rejected the idea that Russian President Vladimir Putin could be prepared to turn the war into a nuclear conflict.
“He probably doesn’t even stop the Ukrainians if he did that – and we would put his economy into such a cryogenic paralysis that Russia wouldn’t come out of it for decades – so he’s not going to do that,” he said.