SCOTTISH Conservative MPs could vote down the Chancellor’s Budget if the UK Government “betrayed” Scotland by including fishing rights in an extended transition period post Brexit.

According to one backbencher, David Mundell, the Scottish Secretary, would “probably resign” if Theresa May pursued this course.

The Herald has been told several of the 13 Scottish Tories have told the party whips they are intent on making a firm stand on the issue and would be compelled to vote against the Government if it opposed their wishes.

Asked if this could include voting down Philip Hammond’s Budget, one Scottish Tory MP said: “Anything is possible.”

The 10 Democratic Unionists, who prop up the Conservative Government, have already threatened to vote against the Budget – which in theory could topple the May administration – over the prospect of Northern Ireland remaining in the single market with a border down the Irish Sea.

While the Prime Minister has stressed she is not proposing any extension to the 21-month transition period, equally she has not ruled one out.

It has been claimed a few days ago her War Cabinet of 11 senior ministers privately talked through the prospect of a transition period extension without the knowledge of the full Cabinet. Mr Mundell is not a member of Mrs May’s inner team.

At the EU summit on Thursday, Jean-Claude Juncker, the European Commission President, said a transition period extension would afford both sides more time to agree a trade deal and so avoid a backstop; he added it "probably will happen".

When the Scottish Secretary got wind of this possibility, he telephoned Downing Street and spoke to a top official, making clear his deep concern.

A source close to Mr Mundell insisted there was no threat of resignation and suggestions to this effect were being “overcooked,” noting how there was at present no clarity about an extension. “It's just an idea that has not been fleshed out," he said.

However, one MP said there was deep alarm among Scottish Conservatives.

“This is a red line for us,” he declared. “If fishing were included in the extension period – and Brussels would want it to be - there is absolutely no way we could support it. It would mean us voting against the Government and David Mundell would probably resign.”

The backbencher said the Government had to know the 13 Scottish Conservative MPs were “willing to flex their muscles if we are pushed into a corner”.

He added: “Fishing is a totemic issue in Scotland and I can see myself going through the lobbies against the Government if need be; which is not something I ever thought I would hear myself say.”

Another Scottish Tory MP was equally adamant that he too would vote against the Government if fishing were not exempted from a transition extension. “Yes, I would. I have already told the whips. All 13 of us feel the same way. I don’t know of any colleague who would not hold the line on this.”

A third MP pointed out how the Government had initially given its word that the UK would take back control of its fishing waters from December 2019 but then it was pushed back to December 2020 due to the transition period; any further extension to December 2021 would cost the party seats in Scotland. “People would rightly feel betrayed by such a move,” he declared.

Simon Collins of the Shetland Fishermen’s Association made a similar point, saying the industry should “not be let down again” as it was under the 1970s Heath Government and that the UK Government had given it a “cast-iron guarantee” that Britain would become an independent coastal state on Brexit Day and leave the EU’s Common Fisheries Policy.

“In March this year, we were told there is going to be an implementation period and you’re stuck shadowing the CFP until the end of 2020. We didn’t like that at all. That was a whiff of betrayal already…If you go back again and say it might not be until December 2020, why should people believe anything the Government says if they carry on like that?”

Earlier, Jeremy Hunt accepted the issue of the transition extension was “very difficult” but suggested it might be helpful to seal a trade deal.

The Foreign Secretary called on Tory colleagues to unite behind the PM as the EU27 were united behind Michel Barnier, their chief Brexit negotiator, to “maximise” her chances of success.

Meanwhile, Tory Remainer Nicky Morgan, the former Education Secretary, said the Conservatives had been put under "existential strain" by Brexit and some friendships and relationships would "never be healed".