DAVID Mundell has insisted Tory rebels who vote down Theresa May's deal risk putting in place a second referendum and reversing Brexit.

The Scottish Secretary said MPs might decide to push forward with another vote rather than crashing out of the European Union without an agreement.

It comes amid reports the Prime Minister is considering delaying Tuesday’s vote on her EU divorce deal in the face of almost certain defeat.

Meanwhile, Cabinet ministers including Boris Johnson have refused to rule out leadership bids in the ensuing crisis.

Speaking on the BBC’s Sunday Politics Scotland show, Mr Mundell said claims Mrs May's deal could be renegotiated are “false”, and said he would find it "extremely difficult" to serve in the Cabinet under Mr Johnson.

He added: “Mr Johnson and I don't agree on a whole range of issues and I don't see myself being able to serve in that way."

He said No Deal is the “default” option if MPs vote against Mrs May’s proposals – but suggested a second referendum is also on the table.

He said: “No Deal is the default. So if we don’t agree the deal on Tuesday, where we’ll be heading is No Deal.

"That’s the default, that’s in law, and you can’t get round that by simply saying we don’t want No Deal. You have to put something else in place.

“So Parliament might – and I don’t take this position – but Parliament might put another referendum in place, and that ultimately could lead to a reversal of the result.”

The Scottish Secretary rejected suggestions Mrs May might put off Tuesday’s vote and go back to Brussels in a bid to secure further concessions.

He said: “The Prime Minister told me yesterday that the vote will go ahead, so that is the basis on which I’m proceeding.

“I don’t see any advantage in delaying the vote. People have to make up their minds on this issue, then the outcome of the vote has to be assessed.

“The Prime Minister will be going to Brussels with the backdrop of whatever has happened in the vote.

“But she’s certainly not intending to go to Brussels ahead of the vote.”

He added: “A lot of people have the fanciful idea that Santa is going to turn up and put a new deal in their stocking. That’s not going to happen. It’s this deal, no deal or uncertainty.

“There’s a whole range of uncertain things that could happen. Nobody wants that. People out there – they want this sorted. And it’s incumbent on MPs to do that.”

Mr Mundell made the comments ahead of a stormy week in Westminster, as the fate of Brexit, and of the wider country, hangs in the balance

He said it was “simply not possible” to scrap Mrs May’s controversial backstop plans and push ahead with the rest of the deal, as suggested by Boris Johnson.

The backstop, which aims to prevent a hard border on the island of Ireland, would kick in if the UK failed to secure a wider trade deal with the EU by the end of the transition period.

However it has been widely criticised by Brexiters, who claim it would leave the UK tied to Brussels’ rules indefinitely.

Mr Mundell said: “It’s simply not possible to have a withdrawal agreement without the backstop. Mr Johnson knows that. I mean, he was in the Cabinet until relatively recently.

“I don’t understand how he’s come to that conclusion. There won’t be a withdrawal agreement without a backstop.”

It came as Mr Johnson – who has been a persistent critic of Mrs May’s deal – refused to rule out challenging Theresa May for the Tory leadership.

The former Foreign Secretary insisted the Brexit deal should be negotiated, adding: "It's a relatively simple job to do.

"We can have a withdrawal agreement that does not contain the backstop. We can do much, much better than this."

Elsewhere, Mr Mundell said he had made a “judgement” that Mrs May’s deal was the best option to protect the integrity of the UK.

Both he and Scottish Tory leader Ruth Davidson previously threatened to resign if Northern Ireland was treated differently to the rest of the UK.

He said: “You resign from the Government if you can’t support the proposal the Government’s taking forward.

“I can support it because I believe that the alternatives to that proposal are much, much worse – and much worse for the integrity of the United Kingdom.

“A No Deal Brexit would be catastrophic for the Scottish economy, for jobs, for people here in Scotland.”

He added: "I have had to make a judgment on what is in the backstop, which is a temporary measure which may never come into force, and what risk that causes to the integrity of the United Kingdom, compared to crashing out of the EU in less than four months’ time, which I regard as the most significant threat to the integrity of the United Kingdom."