Nicola Sturgeon will have a Brexit showdown with Theresa May this afternoon as David Mundell signalled he would resign if the Prime Minister tied UK fishing to EU rules beyond the December 2020 transition period.

In another critical week for her premiership, the Prime Minister continues to face the threat of a no-confidence vote by Tory rebels; last night she held private talks with Brexiteers and former Cabinet ministers Iain Duncan Smith and Owen Paterson to quell the rebellion.

However, by this morning, the 48 signatures needed to trigger the ballot were still not forthcoming; only 26 Conservative MPs have publicly admitted to handing in letters calling for a vote.

This morning, Mrs May chaired Cabinet, facing down any calls from colleagues to tweak the withdrawal agreement. She will visit Brussels tomorrow to meet Jean-Claude Juncker, the Commission President, to try to sign off the future framework deal ahead of Sunday's special EU summit.

The First Minister, before her scheduled meeting with Mrs May today, claimed the PM’s insistence that the choice before MPs between her draft withdrawal agreement and a no-deal was a false one.

“This is a time for grown-up, responsible governance in the public interest,” declared Ms Sturgeon, “something which has been sadly lacking to date as the Tory Party has continued to put its own interests ahead of the interests of jobs, communities and businesses.

“It mustn’t be an option between frying pan or fire but it is now incumbent on all of us who oppose that false choice to propose a workable alternative.”

The FM is also due to have a private Commons meeting with Labour leader Jeremy Corbyn this lunch-time to push a united front for continued single market and customs union membership as an alternative to a no-deal scenario.

To the same end, she is also due to hold talks with Sir Vince Cable, the Liberal Democrat leader, Liz Saville Roberts for Plaid Cymru and Caroline Lucas for the Greens. The issue of pushing for a People's Vote is also likely to come up.

Last night, Labour’s Sir Keir Starmer said Labour could even try to bring down the Conservative Government to block a no-deal Brexit.

The Shadow Brexit Secretary told the Parliamentary Labour Party he was prepared to “work with all sides” to prevent the UK crashing out of the EU without a deal.

"There will be opportunities to make the majority against no-deal heard. Motions will be tabled, amendments will be pressed and a no-confidence vote can be triggered.”

If a no-confidence motion passed, a new government would have to be formed within 14 days and win its own confidence vote; if it failed, there would be a general election.

In Whitehall yesterday, the SNP’s Michael Russell, attending his 15th Joint Ministerial Committee, brushed aside questions about Ms Sturgeon calling for a second independence referendum, insisting the “first priority” was to stop what he branded a disastrous withdrawal agreement.

Claiming UK ministers had lost “all sense of reason,” the Scottish Government’s Constitutional Relations Secretary declared: “It is an emergency for all the nations in these islands to stop this deal.”

But, post the fiery JMC, Mr Mundell launched a withering attack on the motives of the SNP leadership.

Claiming the worst possible outcome for Scotland and the UK would be a no-deal, he declared: “What the SNP are looking to achieve is chaos...because they believe that’s the best way to take forward their independence agenda.”

He added: “If there is a majority in the House of Commons against this deal, we risk a no-deal Brexit and a disastrous outcome for Scotland.”

Mr Mundell was also pressed on the issue of including UK fishing rights in any extension to the 21-month transition period.

“I'm very clear we can't extend the period and include the Common Fisheries Policy within that extension and I could not support that,” he insisted.

Asked if he would therefore resign if Britain remained bound by CFP rules beyond December 2020, the Secretary of State replied: “I'm not playing into the resign soap opera. What I'm focused on is outcomes and we want the best outcome for Scotland."

Earlier on Monday, the PM, following a speech to the CBI, pointed to the possibility of extending the transition period but noting how this should not go beyond the General Election scheduled for June 2022.

While the draft withdrawal agreement simply points to both sides using their “best endeavours” to get a post-transition agreement on fishing by June 2020, No 10 admitted the issue of UK fishing rights would be part of the negotiations.

“These are all things that would have to be discussed,” said a spokesman.