Rory Stewart, the surprise star of the Tory leadership election, has said he would help organise an “alternative parliament” in order to stop a no-deal Brexit if the new prime minister tried to prorogue parliament in order to bypass MPs’ wishes.

The MP for Penrith and the Border, whose campaign featured a series of walkabouts around the country to gauge public opinion on Brexit, also accused fellow no-deal opponents who are now supporting Boris Johnson of a destructive pessimism about their ability to change the Conservative party, or even win the next election.

Stewart said a former Speaker, such as Betty Boothroyd, could be enlisted to oversee a parliament continuing to sit in defiance of Boris Johnson if he presses ahead with a no-deal Brexit by seeking to prorogue the Commons, or to use some other “constitutional manoeuvre which means whatever legislation parliament tries to pass does not bind his hands”. He said any plan to prorogue parliament, an option still entertained by Johnson, would be a constitutional outrage.

“I think it does not work. I would simply work with colleagues simply to organise another parliament across the road. That sounds quite Civil War-ist, but that is what happened in 2002 when Blair tried not to have a vote on the Iraq war. MPs were invited to Church House, and Blair backed down. I got into a lot of trouble when I first proposed this, though it’s just a fact that parliament is not about [the] building. We can certainly find a retired Speaker to chair this.”

Discussing his plans to fight no deal, he said there is no majority in parliament for an option which he said would knock back economic growth over the next years.

“So I, David Gauke, Ken Clarke and, I guess, most of the people that supported me for the leadership will vote against a no-deal Brexit, and Johnson has only got a majority of two.”

Stewart, a foreign office minister during Johnson’s foreign secretaryship, also had a scathing assessment of his former boss’s leadership skills, saying: “He likes to be popular. I remember I had been pushing our ambassadors to be much more brutally honest about failure and the the weakness of British positions in their countries and he said: ‘Rory, I used to captain rugby teams and that is not how you do it. You say to them: “It is great, we can do this. We are great.”’ It’s the only time he told me off.”

Stewart continued: “I had said I did not want to receive any more telegrams saying ‘Another win for global Britain’ and he told me off: ‘No, you have got to build their morale and make them feel pumped up and feeling it’s going to be great. The more they say it is going to be great, the greater it is going to be.’”

Stewart commented: “My disagreement would be that international trade negotiations are not like a rugby match. It might work in 80 minutes and pump people up, but you cannot do tariff schedules on the basis of a rugby match.” Trade negotiations cannot be won on the playing fields of Eton, he said.

Admitting he is just starting to think through the idea of a centre movement led from within the Conservative party but open to others, he says: “There is a huge gaping hole in British politics. People are yearning for a centre ground somewhere between Nigel Farage and Jeremy Corbyn.

“That is, for me, a lot of Labour and Lib Dem voters prepared to listen. My polling is strong among under-45s, in London and in Scotland. It is difficult to put a number on it, but I think there are tens of thousands, possibly even hundreds of thousands, who would have an appetite for that kind of movement or party.

“The centre has been allowed to be seen as a weak opt-out, a grey fudge, an averaging-out, rather than an independent place which has its own energy.”

He rejects the decision of some modernisers, such as the health secretary Matt Hancock and former chancellor George Osborne, to support Johnson, describing them as oddly pessimistic.

“What he and George Osborne [have] also done is rather extraordinary. Osborne is also theoretically a rampant remainer, but has just backed a no-deal Brexiter. I guess what they have all decided is that what matters is to be inside the tent peeing out, rather than being out of the tent peeing in, and someone like me is vaunting my principles but effectively surrendering any ability to change the world, and I am going to end up an an impotent backbencher.”

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He continues: “I feel quite sad about people who are oddly pessimistic about their ability to change the world or to stop Boris winning, because of their inability to change the minds of Conservative association members. They are even pessimistic about winning another election. There is a fin de siècle feeling to the whole thing.”

He contrasts their actions with figures like David Gauke, the justice secretary, who has also rejected Johnson’s populism. “People like David, they have completely refused to make compromises. He is a real example of someone deeply, deeply loyal, a very practical politician, and he had reached the state where he decided: ‘I am not going to put up with this any more and I am going to speak the truth.’ He has thrown himself into the project of just telling the truth, and feels he is taking on populism.”

Stewart challenges the theory that politics has become so polarised by Brexit that his politics will gain no traction. He says: “The pollster John Curtice says that this this not just a issue of party members. It is question of the whole of British politics – that what was once a bell shape has turned into a U shape in which the extremes are popular.”

“Curtice would essentially say that I am fishing in a very shallow pool. I am not sure that is true or how permanent this state is.”

He insists he will resign as international development secretary if Johnson wins. “Unfortunately, the first thing that happens if Johnson wins is that I will lose my job, even though my last nine years had been spent building up to this.”

Instead, he says he will restart his “Rory walks” tours around the country, retelling conversation into an iPhone in social media messages that sometimes reach hundreds of thousands of people. He admits, with his trademark candour: “I am still thinking this through.”