EU chief Brexit negotiator Michel Barnier has called Boris Johnson’s first speech to MPs “rather combative”.
He also said eliminating the Northern Ireland backstop, as the new UK PM proposed, would be “unacceptable”.
The backstop is a mechanism to avoid physical checks on the border between Northern Ireland and the Republic of Ireland in case of a no-deal Brexit.
Mr Johnson said alternatives could be introduced, but the EU has ruled out renegotiating the issue.
In his first statement to the House of Commons since becoming prime minister, Mr Johnson said: “No country that values its independence, and indeed its self-respect, could agree to a treaty which signed away our economic independence and self-government as this backstop does.”
Responding to questions from MPs, he said he was committed to “getting rid” of the backstop, describing it as “divisive” and “anti-democratic”.
“[It] poses that appalling choice to the British government and the British people – to the United Kingdom – of losing control of our trade, losing control of our regulation or else surrendering the government of the United Kingdom,” he added.
In a note sent to EU leaders’ aides on Mr Johnson’s statement, Mr Barnier wrote that, despite disagreements over the backstop, the EU was prepared to “work constructively, within our own mandate”.
He added it was prepared to analyse “any UK idea on withdrawal issues that are compatible with the existing withdrawal agreement”.
However, he insisted that getting rid of the backstop was “of course unacceptable and not within the mandate of the European Council”.
Concerning the possibility of a no deal, he said it would not be “the EU’s choice” but added “as suggested by his [Mr Johnson’s] rather combative speech, we have to be ready for a situation where he gives priority to the planning for ‘no deal’, partly to heap pressure on the unity of the EU27.”
“In any case, what remains essential on our side is to remain calm, stick to our principles and guidelines and show solidarity and unity of the 27.”
He added that he would “remain available throughout the summer for talks with the UK”.
Downing Street did not respond directly to Mr Barnier’s comments, but the prime minister’s official spokesman said the PM would be “energetic in the pursuit” of a deal.
But he added that Theresa May’s withdrawal agreement had “been rejected three times” by MPs and was “clearly not acceptable to them”.
However, at a news conference in County Donegal, Irish Prime Minister Leo Varadkar said, “without the backstop there is no withdrawal agreement, there’s no transition phase, there’s no implementation phase, and there will be no free trade agreement until all those matters are resolved”.
In a phone call with Mr Johnson, EU Commission President Jean-Claude Juncker reiterated the EU’s position that the withdrawal agreement was the best one possible. He also said the commission would be available over the coming weeks if the UK wanted to hold talks.
Meanwhile, he is coming under pressure from one of his own MPs to introduce a law guaranteeing the rights of EU citizens living in the UK.
The prime minister said EU citizens would have “absolute certainty” of their right to live and remain in the country.
Alberto Costa – who resigned as a parliamentary aide over the issue – welcomed the statement, but said the promise should be “underpinned by law”.