Michel Barnier, the European Commission's chief Brexit negotiator, issued a hardline message ruling out any compromise that would “undermine our single market which is one of the EU's biggest achievements”.
“It proposes to apply EU customs rules without being part of the EU's legal order,” Mr Barnier wrote.
“Thus, the UK wants to take back sovereignty and control of its own laws, which we respect, but it cannot ask the EU to lose control of its borders and laws.”
The article comes hard on the heels of reports that the German chancellor is ready to press for a softening of Brussels’ red lines, to make it easier for the embattled Ms May to secure a deal.
British officials believe Ms Merkel – after a positive meeting with the prime minister earlier this month – now wants Mr Barnier to settle for a vague blueprint for future economic ties.
The approach – dubbed a “blind Brexit” by some – could allow the UK to ease into the post-Brexit transition period without making hard choices, provided the Irish border dispute is settled.
Cabinet ministers have mounted a summer charm offensive across the continent, in the belief that EU capitals are their route to success – with Ms May herself holding talks with Emmanuel Macron in France on Friday.
However, in his article, Mr Barnier sought to stress that he was not increasingly isolated, but instead was working with the full backing of EU national leaders.
“The European Council – the 27 heads of state or government – as well as the European Parliament have often recalled that these economic foundations cannot be weakened,” he said.
A European Commission source said the article did not represent any shift in position, or that it was meant as a riposte to reports about Germany’s stance.
Despite his rejection of the Chequers plan, Mr Barnier insisted: “I remain confident that the negotiations can reach a good outcome. It is possible to respect EU principles and create a new and ambitious partnership.”
The negotiator also stated a willingness to be flexible about the text designed to ensure no hard border in Ireland – while warning it remained “the biggest risk” posed by Brexit.
The EU's suggested solution – effectively keeping Northern Ireland in the customs union and part of the single market – has so far been rejected by the UK as “annexing” part of the UK.
Mr Barnier said the EU was “ready to improve the text of our proposal with the UK”.
But he warned: “Since we will not know what the future relationship will bring by autumn 2018, we need to have a backstop solution in the withdrawal agreement.”
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