THERESA May could be forced into a hard Brexit unless she gives European boats access to British waters, Emmanuel Macron has warned.

Asked what bargaining chips the EU has in future fishing negotiations, the French President told reporters that Brussels had the power to leave the UK stuck “long-term” in the controversial backstop, effectively keeping Britain in the customs union.

“We as 27 have a clear position on fair competition, on fish, on the subject of the EU’s regulatory autonomy, and that forms part of our lines for the future relationship talks, which is a lever, because it is in our mutual interest to have this future relationship,” he said.

“I can’t imagine that the desire of Theresa May or her supporters is to remain for the long term in a customs union, but to define a proper future relationship which resolves this problem.”

Earlier, German Chancellor Angela Merkel said the talks on fisheries were “undoubtedly going to be an area where negotiations are going to be tough”.

A third paper to accompany the Withdrawal Agreement and the Political Declaration, agreed on Friday night and obtained by the Politico website over the weekend, said an agreement on fishing would be a “matter of priority,” and should build on “existing reciprocal access and quota shares”, and that such an agreement “should be negotiated well before the end of the transition period.”

Tory MPs have expressed concerns. Ross Thomson said the EU was trying to maintain the “damaging” Common Fisheries Policy (CFP). He told BBC’s Sunday Politics Scotland programme: “That [statement] does deeply trouble me.”

The Aberdeen South MP added: “It may not be called that, but if it looks like the CFP and behaves like the CFP then it is the CFP and that’s why we need to resist it as we go into these negotiations.”

May said her deal would put the UK in “full sovereign control of our waters” and that fisheries would not be “tied to any other aspect of our economic partnership” with the EU.

Scotland’s First Minister Nicola Sturgeon added to the pressure on Scottish Secretary David Mundell, who has threatened to resign if the UK remains tied to the CFP at the end of the Brexit transition period in 2020.

She tweeted: “I’ll be interested to hear David Mundell explain how this – ‘existing reciprocal access and quota shares’ – can be squared with the promises made to the Scottish fishing industry. (Hint – it can’t).”

Shadow Scottish Secretary Lesley Laird wrote to Mundell asking him to clarify his position. “I do not need to tell you that this is a clear breach of your red line on fishing,” she said. “I would therefore be grateful if you were able to clarify that you will not be voting for the deal on this basis.”

Mundell showed no sign of standing down, though he also seemed to show little enthusiasm for the package. It was, he tweeted, all that was available: “There is no credible alternative deal on offer, only the threat of chaos or more division.”