The European Union has promised to work with Boris Johnson to achieve an “orderly” Brexit – but that would mean ratifying a Withdrawal Agreement the incoming prime minister has already declared dead.
Brussels has repeatedly said it will not reopen the deal but Mr Johnson has said fundamental changes are needed.
European Commission vice-president Frans Timmermans stressed that the EU’s position on the issue was clear.
Mr Timmermans said a no-deal Brexit would be “a tragedy for all sides, not just for the United Kingdom”.
“We are all going to suffer if that happens,” he told reporters in Brussels.
In a reference to the two versions of Mr Johnson’s article in which he finally came out in support of Brexit, Mr Timmermans said: “He took a long time deciding whether he was for or against Brexit and now his position is clear.
“I think the position of the EU is also clear: the United Kingdom reached an agreement with the European Union and the European Union will stick to that agreement.
“We will hear what the new prime minister has to say when he comes to Brussels.”
The EU’s chief negotiator Michel Barnier also stressed that the Withdrawal Agreement was the basis for an orderly withdrawal.
On Twitter, he said “we look forward to working constructively” with the new prime minister “to facilitate the ratification of the Withdrawal Agreement and achieve an orderly Brexit”.
He added that the EU was prepared to “rework” the Political Declaration setting out the aims for the future relationship, something that Brussels has already indicated it would do.
Mr Barnier will meet MEPs on the European Parliament’s Brexit Steering Group on Wednesday to discuss Mr Johnson’s approach.
Guy Verhofstadt, the parliament’s Brexit co-ordinator, said he was “looking forward to defending the interest of all Europeans”.
Mr Johnson’s political style was criticised by Lithuania’s European Commissioner Vytenis Andriukaitis.
In a blog post entitled “Boris, you are wrong”, he claimed that “fake facts” of the type used by the new Tory leader were killing democracy.
Mr Andriukaitis said: “A functioning democracy demands discussion of us. Using whatever means to win political battles just does not fit the bill.
“Boris Johnson ‘virtuoso’ in democracy is the example of this in action – where priority is given to the objective alone and not the means of obtaining it.
“On this shaky path almost anything is allowed: cheap promises, simplified visions, blatantly evident incorrect statements on ‘EU imposed’ food safety standards.
“Can democracy survive this type of politics? My take is that democracy chooses only those principles that derive from it, defend it and legitimize it. The ones that stem from ‘fake’ facts are killing it.”
He compared Mr Johnson to Boris Yeltsin during the turbulent years of his leadership in Russia – the origin of the “Boris, you are wrong” phrase used in the blog.
“It is a different Boris, of course, but there was something in the way of doing politics that was similar: many unrealistic promises, ignoring economic rationales and rational decisions.”
In a message to Mr Johnson, he added: “I can only wish him luck in ‘taking back control’, spending more money on the NHS, swiftly concluding new trade agreements.
“In other words, I hope and wish that he does not give anyone a reason to use the quote ‘Boris, you are wrong’ against him.”
The leader of the opposition Jeremy Corbyn warned of the dangers of a no-deal Brexit and called for a General Election as fewer than 100,000 unrepresentative people had decided the new Prime Minister of Britain.