Boris Johnson taking over at No 10 would be “the end of the road” for several Conservative MPs who would defect to the new centrist Change UK party, its leader says.

Heidi Allen, a former Conservative, predicted “a number” of her former colleagues would follow her across the Commons if the arch-Brexiteer – or someone like him – succeeded Theresa May as prime minister.

Ms Allen said she had studied the workings of the proportional voting system to be used on 23 May, adding: “I don’t think there’s a risk of that.

“We will poll differently in different parts of the country but I think we stand every chance of doing very, very well in certain regions.”

Mr Johnson remains the bookies favourite to reach Downing Street, in a contest already effectively underway despite Ms May’s determination to stay until she wins Commons approval for a Brexit deal.

Grassroots Tories hope to topple her by triggering an extraordinary general meeting of their national convention, while Conservative MPs continue to gather signatures in a show of strength.

The Independent revealed that Mr Johnson has grabbed himself a coveted new office big enough to accommodate a campaign team, at the heart of the Westminster estate.

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If he reaches a final pairing of candidates, picked by the MPs, he will be the favourite when the ultimate choice is made in a ballot of the party’s 120,000 members.

Asked about that prospect, Ms Allen told The Financial Times that disaffected Tories saw preventing the election of a hard Brexit supporter as party leader as “an absolute red line”.

“If the party actively chooses someone like Boris Johnson, or whoever it might be, then that will tell them, I think, that the game really is over,” she said.

“I think that would potentially be the end of the road for a number of Tory MPs.”

The South Cambridgeshire MP pointed to the heartache of switching as a reason for the lack of recent defections to her new party, born out of the 11-strong Independent Group of defectors from the Tories and Labour in February.

She also acknowledged Change UK had to overcome the hesitation of MPs whose opposition to Brexit clashed with the majority of their local voters.

“There are some MPs who want to come and join us but they represent Leave constituencies,” Ms Allen said.