DAVID Mundell has given the clearest indication yet that he would quit as Scottish Secretary if the UK stayed tied to EU fishing policies and quotas beyond the end of the transition period in December 2020.

The Scottish Secretary’s signal came as Downing Street suggested that the issue of whether or not fishing rights would be included in any transition extension was part of the ongoing negotiations.

Extending the 21-month transition period as an alternative to the deeply divisive issue of the backstop was raised by Theresa May at an EU summit last month. At the time the Prime Minister made clear this was not something she wanted to happen but, if it did, it would only last a few months beyond December 2020.

Yet, because fishing quotas are set in December for the following 12 months, any transition extension that included fishing, would mean the UK not taking back full control of its fishing waters until December 2021 at the earliest.

However on Monday, Mrs May and Greg Clark, the Business Secretary, appeared more amenable to the option after Michel Barnier, the EU’s chief negotiator, spoke about a possible extension to December 2022.

Mrs May told the CBI’s annual conference that any transition extension would have to end well before the next General Election; due in June 2022. But her remarks appeared to point to more than just a few months.

Mr Clark noted how extending the transition would be beneficial to the business community, which did not want more than one change from the current position; a backstop would involve two.

The PM’s spokesman explained if both sides were looking at a few weeks or months to complete the trade deal, then “this is a decision to be taken around June 2020 and…it will be at that point in time we would seek a short extension of the implementation period as opposed to going into the backstop”.

Asked if this was now the PM’s preferred option, he replied that it would “very much depend on the facts at the time” but stressed the key point was none of this was expected to happen as the PM believed the future relationship would be in place by January 2021.

Asked if the conditions of any extended transition period would be different from the planned transition – such as on fishing policy - the spokesman replied: “These are all things that would have to be discussed.”

Last week, the issue was raised in the Commons by Cornish Conservative MP Sheryll Murray to which the PM replied: “We have not changed our position, which is that, as of December 2020, the UK should be an independent coastal state able to negotiate the issue of access to its waters for the following year and, obviously, for thereafter.” MPs noted the word “should” rather than will.

Mr Mundell and his 12 fellow Scottish Conservative MPs have written to their party leader, threatening mutiny if UK fishermen remain tied to the hated CFP beyond December 2020.

“We are leaving the CFP and becoming an independent coastal state in 2020. If the implementation period is extended, as far as I’m concerned that will have to remain the case,” declared the Scottish Secretary. “I'm very clear we can't extend the period and include the CFP within that extension and I could not support that.”

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

While both sides have delayed a decision on what happens on fishing rights beyond December 2020, promising to use their “best endeavours” to sort it out by the summer of 2020, France, in particular, is making clear the future UK-EU free trade agreement will depend on Britain offering access to its waters similar to current arrangements. This, however, would be anathema to the UK fishing industry.