Rory Stewart in No 10 boost after receiving David Lidington's backing

Theresa May’s effective deputy has thrown his weight behind the leadership campaign of Rory Stewart, as the underdog candidate vowed to head efforts to stop a no-deal Brexit.

David Lidington’s endorsement of the international development secretary as Conservative leader and next PM will inevitably fuel Westminster rumours that Ms May herself is backing Stewart – though she insisted she will not say who she has voted for.

In a defiant pushback at frontrunner Boris Johnson, Mr Stewart said he was ready to join “nearly 100” Conservative MPs in voting down a no-deal outcome if he is eliminated by a hard Brexiteer.

Despite picking up momentum since entering the succession battle as a rank outsider and being installed as bookies’ second favourite, Mr Stewart faces a struggle to pass the threshold of 33 MPs’ votes needed to remain in the race after the second round of voting on Tuesday.

He was cautious about his prospects on Monday, saying only that he should get through if people who have offered him their support “do what they say”. But with just 19 backers in the first round, he is in a perilous position alongside home secretary Sajid Javid on 23 and former Brexit secretary Dominic Raab on 27.

The contender finishing last will be kicked out of the race, along with any who fail to overcome the 33-vote hurdle. Mr Raab said he was "quietly confident" of meeting the challenge, despite seeing many hardline Brexiteers fall in behind Mr Johnson.

Alongside Mr Lidington, Mr Stewart has picked up support from ministers Tobias Ellwood and Margot James, former party chair Caroline Spelman and Scottish MP Paul Masterson.

Speaking at a rally for Stewart, Mr Lidington – who, like Ellwood and James, previously backed health secretary Matt Hancock – said there was “a yearning in this country for political leaders who tell it straight to people, who are honest about the difficulties and challenges that lie ahead (and) are willing to listen, who are prepared to get out of the comfort zone and out of the Westminster bubble”.

Reports at the weekend suggested that Ms May might have given her first-round vote to the Penrith & The Borders MP, who is the only candidate to promise another attempt at getting her three-times rejected Brexit deal through parliament.

But the outgoing PM insisted: “I am not backing a particular candidate. I haven’t endorsed a particular candidate.

“I did vote last Thursday. I haven’t told anybody who I voted for and I’m not going to.”

Mr Johnson, who established a commanding lead on 114 votes to his nearest rival Jeremy Hunt’s 43 in the first round, has continued to pick up endorsements, including from former contenders Mr Hancock and Esther McVey.

But Mr Hunt issued a warning to MPs of the dangers of a Johnson victory, telling a backbench hustings that Brussels will not do a deal with someone they do not trust, forcing Tories into an election that would spell “devastation” for the party.

“If we put forward the wrong person there will be no trust, no negotiation and no deal,” said the foreign secretary.

He suggested that some MPs have now resigned themselves to an early election and are offering Mr Johnson their backing as the candidate most likely to be able to salvage a result from it. But he warned: “No campaigning brilliance will head off the devastation we would face from an election before we leave the EU.”

Addressing a hustings of Westminster journalists, Mr Gove committed himself to a vote of MPs on any Brexit outcome, deal or no deal.

“I think it would be a mistake for any prime minister to say they were doing something as momentous – and potentially liberating – as leaving the EU without parliament … having agreed that the prime minister was doing the right thing,” he said.

Mr Stewart to echo senior Tories Philip Hammond and Ken Clarke who could potentially back a vote-of-no-confidence, saying: “I'm not going to take down a Conservative government.”

Questioned by The Independent at hustings in Westminster, Mr Stewart said: “We can stop a no-deal Brexit much more easily than that.

“I, and nearly 100 of my colleagues, would vote to prevent a no-deal Brexit without having to bring down a Conservative government.”

Mr Stewart also ruled out backing a Final Say referendum on Brexit, telling journalists it would be “catastrophic and divisive”.

Stewart backing justice secretary David Gauke seized on a warning note issued by credit rating agency DBRS, which said that the UK’s AAA rating could come under downward pressure from “a significant increase in the likelihood of a break-up of the United Kingdom such that could occur in the context of a no deal Brexit outcome”.

Mr Gauke described the report as a “stark” warning that no-deal would “threaten our precious Union of nations and undermine our financial stability”.