Rishi Sunak has brushed off any suggestion he should "unleash a torpedo" to remind Boris Johnson who's Prime Minister.
The jet-setting former Tory leader has been active on the world stage in recent weeks, meeting politicians in Ukraine and the US despite holding no role in government.
He's been a thorn in Mr Sunak's side, criticising the government for not sending fighter jets to Ukraine amid speculation he's maintaining his profile for a future leadership bid.
Piers Morgan asked Mr Sunak if he was bothered by his predecessor-but-one being "on manoeuvres" and "acting like he's still Prime Minister" - and questioned whether he should be put back in his place.
Downing Street said Mr Johnson has been acting "in his own capacity" when he piled pressure on the government to supply fighter jets to Ukraine after meeting President Volodymyr Zelensky
During an interview on TalkTV Mr Morgan asked Mr Sunak whether the time had come to "unleash a torpedo to remind him who's the Prime Minister?"
But the Prime Minister, whose resignation sparked a crisis which helped to topple Mr Johnson, took a diplomatic approach.
Mr Sunak responded: "No I think it's great that we've got former Prime Ministers and I speak to all our former leaders actually."
Pressed on whether Mr Johnson should be doing what he's doing, the PM replied: "I think all of our leaders have a way of continuing to contribute to public life and that's a good thing... they've got experience that they want to share."
Asked if he had a problem with his predecessor but-one's activities he said: "No, gosh, we've got a long list of previous Prime Ministers and the fact they still want to contribute to public life is great."
Downing Street has highlighted the complexities of training Ukrainian pilots to fly the RAF's Typhoon and F-35 planes.
Mr Sunak argued they "are incredibly sophisticated pieces of equipment that require months if not years for people to be trained on".
But asked if he could rule out supplying them in the future, he said: "No, we're always in a dialogue with our Ukrainian friends about what the right way to support them is."
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.
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.
