ANGRY Scottish Conservative MPs have warned Theresa May that they are prepared to collapse her Government if she reneges on a “fundamental pledge” to give Britain’s fishermen full sovereignty over UK waters from 2021.

The Herald has been told that the group of 13 Scottish Tory MPs feel the issue is “totemic”; that after years, from Ted Heath onwards, when Conservatives were accused of letting down the fishing industry, they could not be seen to be doing so again.

“It’s the position of the Scottish Conservative Group that if we don’t get a guarantee of full control of UK waters after the implementation period is over, then we will be prepared to vote down the final EU Bill,” explained one MP.

Asked if the group was really prepared to see the fall of the May Government over fishing rights, he replied: “That’s how seriously we are taking this issue.”

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Another MP insisted: “We don’t want to bring the Government down, obviously, but the Prime Minister knows the vote will all be about numbers. She must honour her fundamental pledge.”

The Scottish Conservative Group has previously pointed out to No 10 how, with 13 members, it is larger than the 10 Democratic Unionists, who are propping up the Government.

As David Davis, the Brexit Secretary, and Michel Barnier, the EU’s chief negotiator, set out the basis for a transitional deal, which looks set to be ratified by European leaders at their meeting on Thursday, Nicola Sturgeon accused Mrs May of a “massive sell-out” of Scottish fishermen over the deal struck over the 21-month implementation period.

This will mean UK fishermen will have to abide by quotas set by Brussels until December 2020.

The Scottish Fishermen’s Federation said it had been let down as UK fishing communities would be “subject to the whim and largesse of the EU for another two years”.

Ruth Davidson, the Scottish Conservative leader, expressed her own disappointment at the transitional deal but also warned Mrs May: “I should make it clear…I will not support a deal as we leave the EU which, over the long term, fails to deliver that full control over fish stocks and vessel access.”

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John Lamont, the Tory MP for Berwickshire, Roxburgh and Selkirk, asked if he would vote down the final Brexit deal, if it did not include total control of fish stocks and vessel access, tweeted: “Yes.” Later, Ms Davidson added her own tweet, saying Mr Lamont would have her "full support" if that came to pass while her response was liked by David Mundell, the Scottish Secretary.

Their Conservative colleague Ross Thomson, who represents Aberdeen South, asked if it were indeed possible he and his Conservative colleagues could vote against their own Government should full sovereignty over fishing rights not be guaranteed, replied: “We can’t vote to betray our fishermen.”

Douglas Ross, the MP for Moray, summed up the dismay and disappointment of his Scottish colleagues over the transitional deal, saying: “There is no spinning this as a good outcome; it would easier to get someone to drink a pint of cold sick than try to sell this as a success."

One Government insider suggested that Michael Gove, the Environment Secretary, who last week penned an article with Ms Davidson making clear how during the implementation period “we will ensure British fishermen's interests are properly safeguarded," was livid at not being kept in the loop regarding the precise deal Mr Davis had hatched on fishing with Mr Barnier. Tory HQ was also said to have been taken by surprise by the details.

Downing Street denied the transitional deal was a “sell-out”. The PM’s spokesman explained that the original text only contained an unspecified consultation with the EU in advance of setting its total allowable catch limits; now, “specific safeguards” had been agreed relating to the annual negotiations for 2019.

The EU, he pointed out, would have to consult the UK ahead of those negotiations, give a commitment its share of the catch could not be changed and that from December 2020 Britain would negotiate as a fully independent coastal state, “deciding who can access our waters and on what terms”.

Asked if the PM could give a cast-iron guarantee that from January 2021 the UK would have full sovereignty over its fishing waters, the spokesman replied: “Yes. We have been absolutely clear once we leave the EU we will be taking control of our fishing waters and, as the PM set out in her Mansion House speech, ensuring there are fairer rights for UK fishermen.”

Outwith Westminster, Bertie Armstrong for the Scottish Fishermen’s Federation said what Mr Davis had agreed with the EU27 on the implementation period fell “far short of an acceptable deal”.

He explained: “We will leave the EU and leave the Common Fisheries Policy but hand back sovereignty over our seas a few seconds later.”

Mr Armstrong said UK fishing communities did not trust Brussels to look after their interests, warning it: “Be careful what you do or the consequences later will be severe.”

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Claiming the Tory Government held let the industry down, he added: “We expect a written, cast-iron guarantee that after the implementation period, sovereignty will mean sovereignty and we will not enter into any deal which gives any other nation or the EU continued rights of access or quota other than those negotiated as part of the annual coastal states’ negotiations.”