THERESA May has upped the rhetoric with Brussels over Brexit and fishing rights, warning the EU27 that if it did not agree a new deal by the end of 2020, then its fishermen would “not have access to our waters”.

Following the weekend special summit that endorsed the UK-EU Withdrawal Agreement, concerns were raised that the wording linked future trade talks with fishing access rights and that it was still not clear if British fishermen would have to continue to abide by the hated Common Fisheries Policy[CFP] were the transition period extended beyond December 2020.

The Brexit deal said an agreement on future arrangements had to be signed by July 2020.

Emmanuel Macron, the French President, angered UK ministers by suggesting that he could use the issue of EU fishing access rights to UK waters as a “lever” with the threat of delaying a trade deal and pushing Britain into the controversial backstop if Brussels did not get its way.

On Monday in her Commons statement, the Prime Minister faced the accusation from the SNP’s Ian Blackford that she had “sold out” Scottish fishermen; a charge she denied.

Mrs May was adamant that while the EU27 had sought to link fishing access to market access, they had failed and would fail again.

Ahead of her visit to Scotland on Wednesday to sell her Brexit Plan, Mrs May, mindful of the sensitivities on the fishing access issue, sent a letter to Scottish Conservative colleagues, telling them of her “resolute position”: there would be no trade-off by Britain of fishing access for market access.

The PM wrote: “If we can’t reach a new and fair agreement by the end of 2020, then the default position is that the EU vessels would have no access to our waters, so they have an incentive to reach one.”

Interestingly, Mrs May did not rule out including the UK fishing industry in the CFP were the transition period extended beyond December 2020, noting only how this would be “unacceptable” to Scottish MPs.

During a feisty Scottish Questions in the Commons that touched on fishing rights, David Mundell faced more calls to resign over Brexit as he clashed with Labour and the SNP.

Lesley Laird, the Shadow Scottish Secretary, referred to his “red lines” regarding his support for the PM’s deal in relation to fishing access and maintaining the integrity of the UK.

“Unless those things were protected,” explained the Kirkcaldy MP, “he would resign. The Prime Minister has come back with a deal that creates a border in the Irish Sea and sells out Scottish fishermen. May I ask: what he is still doing at the Dispatch Box?”

Mr Mundell hit back, insisting he was standing up for the integrity of the Union.

“When I see Jeremy Corbyn and Nicola Sturgeon caballing about what they are going to do next and no doubt agreeing the keys of 10 Downing Street will be handed over to Labour for another independence referendum, I know I am doing the right thing,” he declared.

The SNP’s Pete Wishart suggested the Secretary of State cut a ridiculous figure with his “resignation-non-resignation business”.

The Perth MP said: “He is like a demented Grand Old Duke of York. He has led his merry band of Scotch Tories halfway up resignation hill and has forgotten whether he is going up or down. Scotland voted overwhelmingly against Brexit and increasing numbers of Scots do not want anything to do with it. If the Secretary of State cannot represent the people of Scotland, will he just resign and get out of the way, for goodness’ sake?”

But Mr Mundell responded by saying there was “no greater expert in the House on being ridiculous than the honourable gentleman, swinging one way and another on every issue of the day”.

He added: “I am quite clear: the United Kingdom voted to leave the European Union and this Government will deliver that.”