Michel Barnier has warned he is “strongly opposed” to the prime minister’s Chequers proposal on future trade with the EU after Brexit, warning the “illegal” offer would “end” the European project.

In what were some of his harshest words yet for the British government’s plans, the EU’s chief Brexit negotiator said allowing Britain to “cherry pick” regulations would have “serious consequences”, adding it would be “much easier” if the UK stayed in the single market.

Mr Barnier’s comments came in an interview with newspaper Frankfurter Allgemeine Zeitung, in which he told European carmakers they would need to shun British manufacturers in order to enjoy low tariffs on their global exports.

“The British have a choice,” he said. “They could stay in the single market, like Norway, which is also not a member of the EU, but they would then have to accept all the regulations and make contributions to European solidarity.

“But if we let the British cherry pick which of our regulations to follow, that would have serious consequences: all sorts of other third countries could insist that we offer them the same deal.

“That would be the end of the single market and the European project. I am often accused of being dogmatic in the UK, but the truth is I'm only protecting our fundamental interests.”

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Mr Barnier’s comments came after Theresa May insisted she would not be pushed around by Brussels in an op-ed for The Sunday Telegraph.

“I will not be pushed into accepting compromises on the Chequers proposals that are not in our national interest,” she wrote. “Neither will I give in to those who want to reopen the whole question with a second referendum... To ask the question all over again would be a gross betrayal of our democracy – and a betrayal of that trust.”

But ex-Brexit secretary David Davis said on Sunday the proposals put forward by Ms May would be “worse than staying in” the EU, and confirmed he would vote against any deal based on the Chequers compromise.

Brexit casualties Show all 10 1 /10 Brexit casualties Brexit casualties Andrea Jenkyns - Resigned from Parliamentary Private Secretary at the ministry for housing, communities and local government role May 2018 - The Morley and Outwood MP said: “We want to see a new relationship with Europe, with a new model not enjoyed by other countries – nothing that leaves us half-in, half-out. “And in order to achieve this, we need to leave the customs union.” Ms Jenkyn’s also said she wished to dedicate more of her time to Parliament’s influential Exiting the European Union select committee, after a series of “unbalanced” reports produced by MPs PA Brexit casualties David Davis - Resigned from Secretary of State for Exiting the European Union role July 2018 - quit following a major row with May over her plans for post-Brexit relations with the EU. Davis resignation letter said: “As you know there have been a significant number of occasions in the last year or so on which I have disagreed with the Number 10 policy line, ranging from accepting the [European] Commission’s sequencing of negotiations, through to the language on Northern Ireland in the December Joint Report. “At each stage I have accepted collective responsibility because it is part of my task to find workable compromises, and because I considered it was still possible to deliver on the mandate of the referendum, and on our manifesto commitment to leave the Customs Union and the Single Market. “I am afraid that I think the current trend of policy and tactics is making that look less and less likely.” He went on to argue that the “general direction” of Ms May’s policies would leave the UK “in at best a weak negotiating position, and possibly an inescapable one”. AFP/Getty Brexit casualties Steve Baker - Resigned from Minister at the Department for Exiting the European Union role July 2018 - Mr Baker, a key Tory figure in the Leave campaign, was David Davis’s main lieutenant at Dexeu, and was hailed as ”courageous and principled” by other Brexiteer Tories as he also left. Reuters Brexit casualties Boris Johnson - Resigned from Foreign Secretary role July 2018 - resigned over May's Chequers plan. In his resignation letter to the prime minister, Mr Johnson said: "On Friday, I acknowledged that my side of the argument were too few to prevail and congratulated you on at least reaching a Cabinet decision on the way forward. "As I said then, the government now has a song to sing. "The trouble is that I have practised the words over the weekend and find that they stick in the throat." Reuters Brexit casualties Conor Burns - Resigned from Parliamentary Private Secretary to Foreign Secretary role July 2018 - A Brexit supporter who worked alongside Boris Johnson stated in his resignation letter: “I've decided it's time to have greater freedom. I want to see the referendum result respected. And there are other areas of policy I want to speak more openly on.” Rex Brexit casualties Chris Green - Resigned from Department for Transport role July 2018 - The Bolton West MP said: "Parliament overwhelmingly decided to give the decision of whether to leave or remain in the European Union to the British people and they made an unambiguous decision that we ought to leave. "I have always understood the idea in 'Brexit means Brexit' is that the final deal should be clear to me and my constituents - that we have, in no uncertain terms, left the European Union. Twitter Ads info and privacy "The direction the negotiations had been taking have suggested that we would not really leave the EU and the conclusion and statements following the Chequers summit confirmed my fears. "I recognise that delivering Brexit is challenging, however I had hoped at tonight's meeting that there would be some certainty that my fears were unfounded but, instead, they have been confirmed. "I have been grateful for the opportunity to serve as Parliamentary Private Secretary and it is with regret that I offer my resignation with immediate effect." PA Brexit casualties Maria Caulfield - Resigned from Conservative Party vice-chair for women role July 2018 - resigned over May's Chequers plan. Lewes MP warned that the direction of travel did “not fully embrace the opportunities that Brexit can provide”. Ms Caulfield said in her letter to the PM: “The policy may assuage vested interests, but the voters will find out and their representatives will be found out. This policy will be bad for our country and bad for the party. “The direct consequences of that will be prime minister Corbyn.” PA Brexit casualties Ben Bradley - Resigned from Conservative Party vice-chair for young people role July 2018 - resigned over May's Chequers plan. The Mansfield MP said: “I admit that I voted to Remain in that ballot. What has swayed me over the last two years to fully back the Brexit vision is the immense opportunities that are available from global trade, and for the ability for Britain to be an outward looking nation in control of our own destiny once again. “I fear that this agreement at Chequers damages those opportunities; that being tied to EU regulations, and the EU tying our hands when seeking to make new trade agreements, will be the worst of all worlds if we do not deliver Brexit in spirit as well as in name, then we are handing Jeremy Corbyn the keys to No10.” PA Brexit casualties Robert Courts - Resigned from Parliamentary Private Secretary role July 2018 - resigned over May's Chequers plan. MP Mr Courts said: “I have taken a very difficult decision to resign my position as [parliamentary private secretary] to express discontent with the Chequers [plans] in votes tomorrow. “I had to think who I wanted to see in the mirror for the rest of my life. I cannot tell the people of Woxon that I support the proposals in their current form.” Getty Brexit casualties Scott Mann - Resigned from Parliamentary Private Secretary role July 2018 - resigned over May's Chequers plan. "I fear elements of the Brexit white paper will inevitably put me in direct conflict with the views expressed by a large section of my constituents. I am not prepared to compromise their wishes to deliver a watered-down Brexit. "The residents of North Cornwall made it very clear that they wish to have control over our fishery, our agricultural policy, our money, our laws and our borders. I will evaluate those principles against the Brexit white paper and ensure that I vote in line with their wishes." Rex

His admission makes it more likely the prime minister will be unable to get any deal based on her plans through the House of Commons, where she has a wafer-thin majority and a number of her own MPs already openly opposing her proposals.

In comments likely to concern Britons employed by the car industry, Mr Barnier warned EU manufacturers that the frictionless imports and exports between the UK and the continent would come to an end after Brexit.

“Outside of the internal market and the customs union, this involves customs formalities and controls that hinder ‘just in time’ production,” Mr Barnier said.