Rory Stewart’s bid for No 10 has received a huge boost after the rank outsider received the surprise backing of David Lidington, Theresa May’s ‘deputy’.

The international development secretary’s camp hailed the move as a “huge vote of confidence not just in Rory’s campaign, but in his ability to deliver as prime minister”.

It came as Tory MPs prepared to vote on Tuesday in the second round of voting, which has been expected to knock Mr Stewart out of the race.

He needs 14 more votes than the 19 he received last Thursday – and to overhaul Sajid Javid, the home secretary, for his leadership bid to remain alive.

Appearing alongside Mr Stewart at a rally, Mr Lidington took a thinly-veiled swipe at the other candidates, as he praised his determination to “tell it straight” and be “honest about the difficulties”

“I am endorsing Rory Stewart to be the next prime minister and leader of the Conservative party,” said the man who deputises at prime minister’s questions.

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

“I think there is 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, as well as the policies and the remedies that they are proposing.”

Mr Lidington also praised Mr Stewart’s “willingness to listen”, adding he was “prepared to get out of the comfort zone and out of the Westminster bubble”.

He had backed Matt Hancock in the first round, but has refused to follow the health secretary’s decision to switch to Boris Johnson, still the overwhelming favourite to reach Downing Street.

His support will help the Stewart campaign convince Tories that the momentum is with their candidate, after his strong showing in Sunday night’s live TV debate.

“Rory is incredibly proud to have the support of David Lidington – the deputy prime minister,” a spokesman said.

Earlier, Mr Stewart stepped up his criticism of a no-deal Brexit by telling The Independent up to 100 Conservative MPs would vote with him to stop Mr Johnson carrying it out.

However, he ruled out joining with Labour to topple him in a vote of no confidence, if necessary, “I'm not going to take down a Conservative government.”

“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 argued.

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