Tory leadership contender Dominic Raab has warned his party will be "toast" and unable to win a general election if it fails to secure Brexit by the end of October.

The ex-Brexit secretary said the party's woeful performance in the European elections and Peterborough by-election underlined the damage already inflicted on the Conservatives.

The stark warning from Mr Raab came as he prepared to participate in a televised debate with four of his rivals to replace Theresa May in Downing Street on Sunday evening.

Clear favourite to succeed the prime minister, Boris Johnson, has refused to attend the Channel 4 debate, and the broadcaster plans to present viewers with an empty podium.

"When people voted, they vote to Leave. We haven't left yet and that's why we're seeing not just the uncertainty for the economy – but also this corrosion of public trust," Mr Raab told Sky News' Sophy Ridge on Sunday.

Who is standing to be the next prime minister? Show all 3 1 /3 Who is standing to be the next prime minister? Who is standing to be the next prime minister? Boris Johnson - 157 votes Foreign Secretary Boris Johnson arriving in Downing Street, London, for a Cabinet meeting where Theresa May will brief on her plans for Brexit before a major speech aimed at helping to break the deadlock in the negotiations with Brussels. PA Who is standing to be the next prime minister? Michael Gove - 61 votes LONDON, ENGLAND - JUNE 07: Britain's Environment Secretary Michael Gove leaves 10 Downing Street on June 7, 2018 in London, England. Prime Minister Theresa May is holding an emergency Brexit cabinet meeting in an attempt to resolve tensions over the UK's Irish border plan. (Photo by Simon Dawson/Getty Images) Getty Who is standing to be the next prime minister? Jeremy Hunt - 59 votes Health and Social Care Secretary Jeremy Hunt leaving Downing Street, London, after attending a Cabinet meeting. PA

"The Tory Party will be toast unless we're out by the end of October. People need to wake up this. I certainly think the Conservatives cannot win an election unless we deliver Brexit."

Mr Raab, who claimed his "underdog" campaign is "building momentum", added: "This race has only just started. I appreciate there is a frontrunner here but I am looking forward to this race."

The former cabinet minister also insisted the other candidates had failed to set out a "credible plan" for Britain's exit from the EU.

"I'm not quite clear what Boris's plan is, but in fairness I'll let him set that out in more detail," he said.

Rory Stewart, a cabinet minister vying for the Tory crown, also attacked Mr Johnson, claiming his plans for Brexit will come "off the rails" once subjected to a detailed examination.

Remaining six candidates in the Conservative leadership contest (Michael Gove, Boris Johnson, Dominic Raab, Rory Stewart, Jeremy Hunt, Sajid Javid) (AFP)

"How is Boris going to deliver Brexit? He keeps saying 'I am going to deliver it'. I don't even know what he believes. He won't talk to me. He won't talk to you. He won't talk to the public," he told BBC1's The Andrew Marr Show.

"We want to know what he believes. The real problem with politics is a problem of trust.

"Nobody has had the chance to question him and as soon as you question him and as soon as I sit down with him and ask the big question - how, how are you going to deliver Brexit? How are you going to get a no-deal through? - Then it begins to come off the rails.

"We are a moderate country, we are not a Trumpian country," Mr Stewart added.

But on Sunday, Mr Johnson's campaign received a boost as Esther McVey, a former cabinet minister who was eliminated from the leadership contest last week, threw her support behind the frontrunner.