Boris Johnson will be "happy, happy, happy" to quit the Government if Theresa May opts for a soft Brexit, his father Stanley has claimed.
Johnson senior, who also let slip that he had discussed with the Foreign Secretary his ambition to be Prime Minister, spoke out as his son repeated that he didn't want the UK to keep paying money to Brussels after it has quit the EU.
As the furious Cabinet row continued, Stanley, a former Remain campaigner and environmentalist, declared that his son felt so passionately about the issue that he was prepared to resign.
"This is such an important issue that I would have thought he would be happy, happy, happy to walk away from the whole thing if that's what he had to do," he told SkyNews.
Johnson junior fuelled the speculation that he could quit if May outlines a softer Brexit in a speech in Florence this Friday, refusing to rule out a resignation and saying only such talk was 'barking up the wrong tree'.
He also revealed that the Johnsons have a secret Whatsapp group and hinted he had discussed his son's political ambitions.
Asked directly by SkyNew's Kay Burley if he had discussed Boris's chances of becoming Prime Minister, his father replied: "We've got Whatsapp nowadays and Whatsapp has encryption on both ends, I'm glad to say. Who knows what people talk about with their parents?
"Why on earth would I...tell you what I talk to my children about? That's just not plausible."
On a fast-moving day, the Prime Minister was forced to interrupt her trip to Canada to slap down her Foreign Secretary, reminding him that her government was "driven from the front".
Her line was a reference to fellow minister Amber Rudd dismissing Johnson as a "back seat driver" after he published a 4,000-word article warning that he would not back a 'soft Brexit'.
Johnson, who is set to meet May in a showdown meeting in New York ahead of her address to the UN General Assembly, repeated his stance on Monday that he would not support any payments to the EU after Brexit in March 2019.
May is due to make a major speech on Brexit in Florence on Friday and Johnson's Daily Telegraph article was seen as a shot across her bows not to follow Chancellor Philip Hammond's drive for a 'status quo' period where the UK would continue to pay billions in return for access to the EU single market.
In TV interviews, he also said that any 'transition' period should be as short as possible, a direct challenge to the Cabinet's new consensus this summer that a changeover of up to two years - to 2021 - will be needed to smooth the exit for business and residents.
Asked if Boris would resign, Johnson senior said: "It seems perfectly clear to me that Boris would say 'look, this is such an important issue, the way we leave, such an important issue i.e. are we going to go on with a transition period which may be two years, may be four years, may be 10 years, or are we going to say No, we voted to leave'?"
"I seem to remember reading Boris's book on Churchill was 52 weeks in the best seller list, don't tell me he hasn't got other things to do."
He added that the Foreign Secretary was well equipped to lead the country.
"Yes I think he definitely could do that without any difficulty. I think he would be absolutely splendid."
Asked if he had fired his shots too early, Johnson's father said: "I'm tremendously happy to have a chance to say something on this topic on air.
"What I would say is if you do shoot yourself in the foot on this issue, then that bullet was very much worth it, and your foot will no doubt recover in due course but gosh it need to be done - because we cannot afford to muddle this Brexit thing up. We have to leave cleanly, we have to leave neatly.
"You can shoot yourself in the foot in a cause worth losing a foot. There are worse places to shoot yourself."
Boris Johnson - who shook hands with Donald Trump at the UN on Monday - gave his own interviews in which he refused to say whether he would quit over his views.
"I think you may be barking up the wrong tree. On the transition period I can see some vital importance of having some clarity and certainty since what all of us want is that it should not be too long," he told the Guardian.
"Let us not try and find rows where there are really not rows. What I am trying to do is in advance of the prime minster's speech ... People want to know where are going. It is good to have a bit of a opening drum roll about what this country can do."