Selecting Boris Johnson or another hard Brexiter to be Conservative party leader is a vote for an early general election, Amber Rudd has warned as the crowded contest finally begins in earnest.

The work and pensions secretary, who has announced she is backing Jeremy Hunt, the foreign secretary, to succeed Theresa May, criticised Johnson’s trenchant attitude towards leaving the EU on 31 October, saying: “I’m afraid that’s not enough if you haven’t got a plan behind it.”

Promises by Johnson, Dominic Raab and Esther McVey to leave without a deal if, as seems likely, the EU refuses to renegotiate the UK’s departure deal, would be blocked by MPs and end up triggering an election, Rudd said.

Asked if a vote for a no-deal candidate was, in effect, a vote for an early election, Rudd told BBC Radio 4’s Today programme: “In my view it is.”

She said: “Members of parliament will find a way to stop no deal and I think that any aspiring leader needs to factor that in.

“I think there could be an election – parliament is a very creative place, and with the assistance of an activist Speaker there will be a way that MPs will find in order to have their voice heard.”

After May formally tendered her resignation on Friday, Hunt, Raab, the former Brexit secretary, and Matt Hancock, the health secretary, will launch their leadership bids on Monday.

Nominations for the post close at 5pm, by which point candidates must have gained at least eight nominations to proceed to the first round of voting. With 11 MPs standing, the expectation is that some will fail to reach the threshold.

Quick Guide Tory party leadership contest Show What happens next in the Tory party leadership race? As she announced on 24 May, Theresa May stepped down formally as Conservative leader on Friday 7 June, although she remains in place as prime minister until her successor is chosen. In a Conservative leadership contest MPs hold a series of votes, in order to narrow down the initially crowded field to two leadership hopefuls, who go to a postal ballot of members.

How does the voting work? MPs choose one candidate, in a secret ballot held in a committee room in the House of Commons. The votes are tallied and the results announced on the same day. In the first round any candidate who won the support of less than 17 MPs was eliminated. In the second round anybody reaching less than 33 votes was eliminated. In subsequent rounds the bottom placed contender drops out until there are only two left. The party membership then chooses between them. When will the results be announced?

The postal ballot of members has begun, and the Tory party says it will announce the new prime minister on 23 July.. Photograph: Neil Hall/EPA

Johnson is the clear favourite, with other leading contenders including Hunt, Sajid Javid, the home secretary, and Michael Gove, the environment secretary. Gove’s chances are regarded to have been badly hit after he revealed he took cocaine while he was working as a journalist and advocating robust measures against drug use.

Rudd said her decision to back Hunt came after the One Nation group of Tory MPs, of which she is part, held hustings events for candidates last week.

“By the end of that double two-hour session it was clear to me that Jeremy was going to be the best person to solve the impasse,” Rudd said.

“We need somebody who faces up to the challenges that we have, who’s not just going to say: ‘Believe in Britain, it’ll be fine’, but has the experience and the plan to try and heal those divisions and actually bring us to Brexit as soon as possible.”

While Johnson had “many great things” about him, Rudd said, his plans for Brexit were too vague: “To me, it’s not enough to say, we’re definitely leaving by 31 October without addressing how you’re going to resolve it.”

She also criticised Johnson’s plan, announced overnight, to slash the higher rate of income tax: “If you want to badge yourself as a one-nation Conservative you focus on tax cuts and investment in infrastructure to help the lowest paid and people in most difficulty in all parts of this country. That’s not what he’s doing.”

Among the outsiders standing for the role is Andrea Leadsom, who recently resigned as leader of the Commons. While she has so far received only four public pledges of support, Leadsom said she was confident of gaining the eight needed.

Quick Guide Tory leadership contenders Show Jeremy Hunt His style is notably technocratic, with few rhetorical flourishes and an emphasis on his consensual approach and long record as a minister, notably during more than five years as health secretary, a traditional graveyard of ministerial careers. Hunt’s attempts to talk up a backstory as an 'underestimated' entrepreneur can fall flat given he is also the son of an admiral and was head boy at Charterhouse. Overall, Hunt’s approach can seem uninspiring and hard to pin down in terms of core beliefs, hence the 'Theresa in trousers' nickname among some Tory MPs – one that is more catchy than accurate (since May herself often wears trousers). In the final round of MP voting Hunt edged out Michael Gove, 77 votes to 75. Boris Johnson Johnson’s progress to Downing Street appeared unstoppable even before an overwhelming victory in the first round of voting among MPs. Most of his colleagues believe it is now all but inevitable that he will be Britain’s next prime minister. His well-disciplined campaign team will continue with their strategy of subjecting him to minimal media exposure, though once the field is narrowed down to two, the final pair will appear in more than a dozen head-to-head hustings for Tory members. The team’s main aim is simply to keep heads down and avoid Johnson creating headlines for the wrong reasons. It may not have worked. Johnson came first in the final round of MP voting with 160 votes.



She also expressed doubt at Johnson’s tax plans, saying he had not taken account of the difficulties of passing legislation without an outright Conservative majority: “I think in reality, in this parliament, it would be impossible to actually get wholesale tax changes through.”

Leadsom termed her Brexit proposal a “managed exit”, a variant on no deal in which she would persuade the EU and parliament to agree some basic issues over citizens’ rights and other areas.

However, she dismissed the idea floated by Raab of proroguing, or suspending, parliament in order to prevent MPs blocking a no-deal departure in October.

“We live in a parliamentary democracy, and bringing the Queen in and trying to do something that shuts down our parliamentary democracy would be entirely wrong.”