JEREMY Hunt has called for unity from Tory colleagues just hours after David Mundell broke ranks to warn Theresa May that neither he nor his Scottish Conservative colleagues would accept any extension to the transition period if it kept Britain in the “hated” Common Fisheries Policy.

The Scottish Secretary’s warning came just days after he and Ruth Davidson made clear in a letter that any move that would put a border down the Irish Sea and threaten the integrity of the United Kingdom would be a “red line” for them. The move was seen as a clear resignation threat.

Any carve-out for Northern Ireland to stay in the single market would inevitably lead to calls from the SNP Government for the same arrangement for Scotland. During Scottish Questions this week, Mr Mundell insisted he would defend Scotland’s place in the Union “until my last breath”.

The Herald understands that when mention was made of an extension to the 21-month transition period, Mr Mundell contacted Downing Street to express his concern to a senior official.

A source close to the Secretary of State said Mr Mundell simply wanted to make No 10 aware of his and his colleagues' fears of what an extension would mean for the fishing industry. He explained that if there were any extension that he would want an exemption for it, so that Britain took back control of its waters at the agreed time in December 2020.

"There is no threat of resignation and there was no letter to the PM. Things are being overcooked. At the moment, there is no clarity about an extension; it's just an idea that has not been fleshed out," said the source.

The Prime Minister has been clear that she is not seeking an extension to the transition period but has also not ruled one out either, saying if there were one, it would only last “a matter of months”.

Indeed, Jean-Claude Juncker, the European Commission President, at yesterday’s Brussels summit said such a move was now likely as it would give both sides more time and space to conclude a trade deal and thus avoid the need for an Irish backstop; the issue that is the last remaining obstacle to a withdrawal agreement.

The current transition period means that the UK will not take back full control of its fishing waters until December 2020. Because the quotas are set in December it means that any extension to the transition period, even for one month, could result in the UK remaining in the CFP for at least another year ie until December 2021; more than five years after the EU referendum vote.

During Commons question-time on Thursday Michael Gove, the Environment Secretary, was asked about the possibility of extending the transition period and said: “It is vital that we leave the European Union at the earliest possible point so that we can ensure that we are outside the Common Agricultural Policy and the Common Fisheries Policy, and that we take back control to ensure that Scotland’s food and drink manufacturers, along with food and drink manufacturers across the United Kingdom, can enjoy the benefits of being global Britain.”

Asked how confident he was that the UK would become an independent coastal state and take back control of its waters in December 2020, the Scot replied: “Invincibly so.”

Bertie Armstrong, leader of the Scottish Fishermen's Federation, has stressed his organisation’s opposition to any extension to the transition period.

“On Brexit Day next year we will become a coastal state but we have now, to our annoyance, agreed to run on the rules of the CFP for an implementation period. That simply cannot be extended, it is not acceptable."

Tory Brexiteer Ross Thomson described a transition period extension as a “dangerous idea,” which would not get support at Westminster.

“It would leave us in a total state of vassalage as we would be obliged to comply with EU regulations while having no say over them. We also lose our rebate but even more dangerously it would tie our fishermen into the hated CFP for longer, which is both destructive to the industry and totally and utterly unacceptable,” added the Aberdeen MP.

David Duguid, the MP for Banff and Buchan, made clear Britain’s membership of the CFP should end in December 2020.

"It is my red line, is the Scottish Conservative's red line. The terms of the implementation period, which will be part of the withdrawal agreement, dictates how we manage fisheries in that period… we will be negotiating as an independent coastal state in December 2020."

But Mr Hunt said the EU proposal to extend the transition period "might help" help seal an agreement on the future relationship.

However, the Foreign Secretary made clear it was not the main issue that needed to be resolved, which remained the so-called Northern Ireland "backstop" intended to ensure there was no return of a hard border with the Republic.

"The substantive area of disagreement is whether we would agree to a backstop which would allow for customs barriers down the Irish Sea - the effective break-up of the United Kingdom - or whether we agree to a backstop which would allow the UK as a whole to stay in the customs union indefinitely," he explained.

"Neither of those are acceptable. Those are two very important matters of principle for the United Kingdom," declared Mr Hunt.

He urged warring Tory MPs to unite behind the PM’s Brexit strategy and rejected claims that the Mrs May had "capitulated" in the face of EU demands.

"The reason why this week has been difficult is because Theresa May has not buckled. She has held firm. She has stuck to her principles," he told BBC Radio 4's Today programme.

"It is precisely because she has not capitulated that we have not concluded this agreement.

"The one thing I would say to my colleagues is that the great strength of the EU in these negotiations is that the 27 EU nations have remained united.

"We now need to do the same behind Theresa May to maximise her negotiating leverage in Brussels and make sure she does come back with that deal that honours the letter and spirit of the referendum decision."

The PM is facing a fierce backlash across the Tory Party against the idea of extending the UK's Brexit transition period.

Former party leader Iain Duncan Smith said such a move would see the UK paying "tens of billions of pounds" extra to the EU.

He said: "I couldn't understand why we would offer to extend the transition period when we still haven't got anything back in return.

"By extending the backstop we are likely to fall straight into the next budget of the EU which will mean tens of billions of pounds extra to be paid across to the EU.

"It would be very hard to tell the British people that we are extending another year or more into the implementation phase, and we're then going to pay tens of billions of pounds over when we actually say we need it for other domestic programmes.

"We are in a negotiation but at the moment it begins to look more like a capitulation than a negotiation.

"We have got to get some steel in our backbone and do something about actually negotiating, rather than saying 'what would you like?'"

Tory MP Nick Boles, who backed Remain in the EU referendum, also opposes moves by Mrs May to extend the transition period.

“So many of the things that she has told us about what she was proposing have turned out either not to be true, or she hasn't stuck to them, so when the Prime Minister first suggested this transition, and remember what this transition is, we stay in the EU in effect, bound by all of its rules, in all of its institutions, paying our normal contribution for two years.

"We have no vote. It's actually worse than being a member of the EU.

"She said, we are going to be in there until December 2020, not to negotiate a deal, to implement a free trade deal that would already have been negotiated.

"Now, what we hear, is that, actually, no, that's not enough time, and, no, we are not going to be implementing.

"We are going to be negotiating that long-term relationship and now we might need a third year.

"That third year would not cost an additional ten billion euros, it might cost 18 billion euros because we'd be in the new budget cycle of the EU where all of the contributions go up.

"That is just an absolute mile from what we were originally told was necessary," he added.

Meanwhile, in a fresh blow to Mrs May's authority, prominent Tory MP Johnny Mercer, who has been seen as a rising star of the party, branded the Government a "shit show".

In response, Mr Duncan Smith said: "It's not the language I would use - but I can understand his frustration.

"What we've seen over the last 24 hours has really made people quite concerned - not only in the party, but in the country."