Conservative leadership contender Rory Stewart has launched a furious broadside at rival Boris Johnson, accusing the former foreign secretary of not being honest about his Brexit plans and challenging him to rule out suspending parliament to force no deal through.

Speaking to The Independent, Mr Stewart said the leadership front-runner was trying to “out-Farage Farage” with an undeliverable plan to renegotiate Theresa May’s withdrawal agreement which was designed to usher in no-deal Brexit but would instead trigger a disastrous general election.

And he blasted Johnson’s “swaggering machismo” over Brexit, which he said risked poisoning the UK’s relations with Europe.

The verbal assault came ahead of the formal launch of the contest to replace Ms May at No 10, with the official line-up of candidates to be confirmed after nominations close on Monday.

Previously-fancied Michael Gove found his campaign mired in controversy over his admission of past cocaine use, with the environment secretary forced to fend off allegations of hypocrisy and deny having lied on security forms when entering parliament and travelling to the US. He dismissed as “foolish” suggestions that he might be barred from going to the US as prime minister.

Who could succeed Theresa May as Conservative leader? Show all 9 1 /9 Who could succeed Theresa May as Conservative leader? Who could succeed Theresa May as Conservative leader? Boris Johnson Former foreign secretary Boris Johnson has long been hopeful, he previously stood in the leadership contest that followed the Brexit vote and has at many times since been thought to be maneuvering himself towards the goal. He remains a darling of the party's right wing, particularly those in the ERG, and is the most popular choice among Tory voters but his leadership bid would be fiercely opposed by many MPs PA Who could succeed Theresa May as Conservative leader? Michael Gove Environment secretary Michael Gove is another member who has long wanted to be leader. He has lately been known for rousing his party in the commons, his recent speeches on the Brexit deal and Labour's no confidence motion have overshadowed the Prime Minister's. He has been loyal to the Prime Minister, partly to shed his reputation as a backstabber who abandoned Boris Johnson to stand against him in the 2016 leadership election Getty Who could succeed Theresa May as Conservative leader? Dominic Raab Former Brexit secretary Dominic Raab has emerged as a favourite to be the Brexiteer candidate in a contest to succeed to Ms May. He displayed a grip on detail in his role as Brexit secretary. When asked recently if he would like to become prime minister he replied "never say never" Getty Who could succeed Theresa May as Conservative leader? Rory Stewart International development secretary Rory Stewart is pitching himself as the sensible candidate, promising to rule out both a second referendum and a no-deal Brexit. He was only recently promoted to the cabinet, previously serving as prisons minister, where he caught headlines with a pledge to resign if he could not reduce levels of violence within a year PA Who could succeed Theresa May as Conservative leader? Esther McVey The former work and pensions secretary announced that she will be standing for the leadership when May leaves. McVey is the first to explicitly state that she intends to stand. She resigned from the cabinet in protest over May's Brexit deal AFP/Getty Who could succeed Theresa May as Conservative leader? Sajid Javid Home secretary Sajid Javid is said to have a plan in place for a leadership race. He made headlines over Christmas when he declared that people smuggling over the English channel was a "major incident" and more recently when he revoked the citizenship of ISIS bride Shamima Begum. Son of a bus driver, he wants the Conservatives to be seen as the party of social mobility PA Who could succeed Theresa May as Conservative leader? Jeremy Hunt Foreign secretary Jeremy Hunt was recently thought to be the favourite in the event of a leadership race as he could sell himself as the man to unite the party. Critics worry that his long stint as health secretary could return to haunt him at a general election. He has reportedly been holding meetings with Tory MPs over breakfast to promote his leadership PA Who could succeed Theresa May as Conservative leader? Andrea Leadsom Following the Prime Minister's second defeat over her Brexit deal, Leader of the house Andrea Leadsom hosted a dinner party at which "leadership was the only topic of conversation", The Times heard. Leadsom ran against Theresa May in the 2016 leadership election before dropping out, allowing May to become Prime Minister AFP/Getty Who could succeed Theresa May as Conservative leader? Priti Patel Former international development secretary Priti Patel is thought to be positioning herself as a contender. One MP told The Independent "she knows she's from the right of the party, the part which is going to choose the next leader, so she's reminding everyone she's there." Patel left the government late in 2017 after it emerged that she had held undisclosed meetings with Israeli officials PA

Meanwhile, Mr Johnson – who picked up backing from cabinet ministers Chris Grayling and Alun Cairns and former Tory deputy chair James Cleverly – broke his campaign silence with a Sunday Times interview in which he said he would withhold Britain’s £39 billion Brexit divorce bill until Brussels agreed better terms for the UK’s withdrawal from the EU.

The threat brought an immediate response from France, where a source close to President Emmanuel Macron said it would be regarded as “a failure of international commitments equivalent to a sovereign debt default” – something that could send the UK’s credit rating tumbling.

Mr Johnson, who is runaway favourite to succeed Ms May, is promising to take Britain out of the EU by 31 October, with or without a deal, but insists that he is confident he can get an acceptable agreement by the Halloween deadline.

He said he would use money set aside for a no-deal Brexit to fund a cut in income tax bills for people earning more than £50,000.

The former foreign secretary told Monday's Daily Telegraph he would raise the 40 per cent tax rate threshold to salaries of £80,000 if he was prime minister, a pledge which was immediately condemned by Labour.

But Mr Stewart told The Independent a new deal was undeliverable by October, meaning that Mr Johnson was effectively setting a course for no deal, which could only succeed if he asked the Queen to suspend parliament through a process known as prorogation” to prevent MPs from blocking it.

“He is expressly committed to no-deal because what he is saying is ‘I will try to negotiate a different deal by 31 October with Europe and if I can’t then I will go to no-deal’," said the international development secretary.

“Well, Europe is not giving a radically different deal by 31 October, particularly not if he is beginning negotiations by saying ‘By the way, I’m withholding the £39 billion and threatening the Irish border’.”

(Rex Features (Rex Features)

With a clear majority against no-deal in parliament, this left Mr Johnson only with the option of prorogation, he said.

“He has no plan for how he is going to get no-deal through parliament – which is what he is promising to do – except by proroguing and he wouldn’t be honest about whether or not he is going to prorogue,” Mr Stewart said.

He also insisted that prorogation should not even be on the table. “It’s outrageous, it’s unconstitutional, it’s illegal, it’s un-British, it’s undemocratic, it’s unpatriotic, it’s unconscionable,” he said.

In a direct challenge to the former foreign secretary, Mr Stewart said: “I want Boris to answer: Will he or will he not prorogue parliament? I want him to rule it out now, and if he won’t rule it out, I want him to say he won’t rule it out. I believe I can win this entire campaign on that single fact. If this man is genuinely countenancing proroguing parliament, or even leaving it as an option, that should be the end of anybody’s candidacy.”

If Mr Johnson declined to rule out prorogation, the 160,000 Tory members who will finally decide the new leader would be faced with a stark choice, he said.

“I am saying to the members, ‘Are you really going to choose somebody whose entire political programme is to lock the doors of the British parliament?’”

Stewart has said he would not serve in a government led by Mr Johnson, and told The Independent that this applied to any candidate advocating no deal. But he said he would not join a no-confidence vote threatened by Labour if Mr Johnson won the leadership. “My vote would be to prevent no deal, not to bring down a Conservative prime minister,” he said.

The Tory leadership contenders

The international development secretary said he was horrified by Mr Johnson’s interview, which confirmed that he had “decided to out-Farage Farage”.

“The principles laid out in his interview in black and white are irresponsible,” Mr Stewart said. “They include promises that can’t be kept, offensive threats that can’t be sustained, they will lead to the poisoning of our politics and the poisoning of our relationship with Europe. And underlying it all is a kind of swaggering machismo which is not in keeping with somebody standing to be leader.

“This is the type of politics that is now emerging in the US and some parts of Europe. It’s not our politics, it’s not our tradition. I’m very confident that when it comes to the members of the Conservative Party, if they are asked to choose between me and Boris, they will not choose that kind of politics.

“You can’t simultaneously ride the horse of Farage and the horse of reality. Those are two beasts that head off in different directions. The problem with that is that if you try to then jump back to reality, if he tried to then pivot to acknowledge the truth – which is that he has no plan on how he is going to leave the EU, no way of getting no-deal through parliament and no plan as to how he is going to negotiate anything with Europe at all – he will simply precipitate a general election, because the fundamental plank of his promise is undeliverable.”

Mr Stewart, who entered the race as a rank outsider but has upped his profile with visits around the country and videos that have racked up millions of views on social media, questioned many Tories’ belief that Mr Johnson must be leader because he can deliver election success.

Mr Johnson’s approach was “a recipe for delay” that would fuel the rise of Nigel Farage’s Brexit Party, while his approach of returning to the deal agreed last November with the option of a fresh mandate from a citizens’ assembly would ensure that the UK left the EU in an orderly way before the next election.

“You can’t win an election by out-Faraging Farage,” he said. “It just doesn’t work. We tested that theory to destruction in 2017.

“The way to defeat the Brexit Party is to get Brexit done and the reason Boris’s pitch doesn’t work is that he can’t get Brexit done. He would be forced into a general election with Brexit not done, against the Brexit Party and with not a single young person voting for him, without the cities voting for him.

“It would be the worst electoral defeat the Conservative Party has suffered for 20 years or more.”

Although he trails better-known candidates in the betting, Mr Stewart believes he has a real chance of joining Johnson on the shortlist of two put to the party membership later this month, if he can ensure he features in planned TV debates. He said he had already secured the eight MPs’ nominations required to enter the race but was “two or three” short of the 17 needed to get through the first round.

He pointed to opinion polls suggesting that he far outstripped the other candidates on various leadership qualities among the relatively small proportion of voters who have actually heard of him.

“By the time I’m in the final two, everybody who’s voting will have heard of me,” he said. “The evidence is that the more people know me, the more popular I am.