Boris Johnson has suggested that Rishi Sunak should "forget about Putin" and send fighter jets to Ukraine, hours after the prime minister said the move was "not practical".
During a surprise visit to Washington, the former prime minister said there was "no case for delay" in helping Ukrainian forces.
When asked about F-16 jets jets on Tuesday Mr Johnson 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."
Mr Sunak's spokesman said on Tuesday it was "not practical" to send the advanced fighter jets that Kyiv has requested because training Ukrainian forces to fly Britain's "extremely sophisticated" planes would take months.
The Prime Minister's spokesman said there was no opposition to allies sending their own jets.
It exposed a potential rift between Mr Sunak and his predecessor, who was on a self-described "mission" in Washington.
One senior Tory backbencher said Mr Johnson's behaviour was sowing "confusion as to who's leading foreign policy".
"It's not about serving the party, it's about serving Boris," said the senior Tory MP. "It's a real shame that this is a person who could be such an asset to the party if he wanted to be, and is now turning out to be a distraction."
Mr Johnson is expected to be on the US east coast until at least Wednesday, as he makes the case to support the Ukrainians "to win this year".
It was unclear whether Mr Johnson had informed Downing Street of his plans in advance.
He was due to address GOP lawmakers at the Capitol Hill Club, a private Republicans social club in Washington, on Tuesday.
The event was being hosted at the club by congressman Joe Wilson, a Republican on the House foreign affairs committee.
Mr Wilson, a staunch Ukraine supporter, has previously proposed installing a bust of Volodymyr Zelensky, the Ukrainian president, in the US Capitol.
But Mr Johnson will face a tougher reception among some Republicans, who now control the House of Representatives and have opposed an open-ended US commitment to Ukraine.
Amid criticism of Mr Johnson's Washington tour, some UK government sources suggested that the former prime minister was uniquely placed to press the case for continued US support.
Mr Johnson continues to enjoy great popularity among American conservatives and has stronger ties than Mr Sunak among his Republican counterparts.
During his visit, he is expected to address the influential Republican Study Committee, the largest ideological grouping within the House of Representatives, which is committed to small government and limited spending.
On Wednesday, he will also discuss the need for "Western unity and support for Ukraine and what more can be done against the threat Russia poses" at the Atlantic Council think tank.
Mr Johnson said his visit was "primarily to recognise and pay tribute to the immense US contribution to the security of the Ukrainian people".
He added: "My mission is to demonstrate that Ukraine will win - and that there is no conceivable case for delay in further supporting the Ukrainians to win this year.
"We should have no fear of escalation when it comes to the provision of weaponry."
On Tuesday, the former prime minister was hosted by Oksana Markarova, Kyiv's ambassador to Washington, at Ukraine House.
Separately, on Tuesday the Biden administration accused Moscow of not complying with the new Strategic Arms Reduction Treaty (Start), the last remaining arms control treaty between the two nuclear powers.
The US State Department faulted Russia for suspending inspections and cancelling talks, but did not accuse the Kremlin of expanding nuclear warheads beyond agreed limits.
"Russia is not complying with its obligation under the new Start treaty to facilitate inspection activities on its territory," said a State Department spokesman, warning that the refusal "threatens the viability of US-Russian nuclear arms control".
