There must be "no going back" on the Brexit agreement struck between the EU and UK last week on citizens' rights, the status of the Irish border and Britain's £39 billion divorce bill, Michel Barnier has said.
The EU's chief negotiator was speaking as MEPs voiced concern over Brexit Secretary David Davis's suggestion that the joint report agreed by Prime Minister Theresa May and European Commission president Jean-Claude Juncker on Friday was "a statement of intent" rather than a legally binding document.
MEPs in Strasbourg gave their overwhelming support, by 556 votes to 62, for negotiations to move on to their second phase, dealing with trade and the transition to a new post-Brexit relationship.
But the motion they passed included a warning that Mr Davis's comments risked undermining the good faith built up in negotiations and said trade talks could only continue if the UK Government "fully respects" the commitments in last week's report.
Leaders of the 27 remaining EU states are now expected to agree at a Brussels summit on Friday that the second phase can begin.
However, a leaked copy of draft guidelines to be discussed at the European Council summit, obtained by the Financial Times, suggests that this will not trigger an immediate start to trade negotiations.
The document indicated that trade talks may not begin until March 2018, with work in the first three months of next year focusing on Mrs May's request for an "implementation period" of around two years following the official date of Brexit in March 2019.
The document suggests that during this transitional period stretching to 2021, the UK will have to comply fully with EU trade policy and not strike its own deals, even though it is due to fall out of EU agreements with more than 50 other nations when it leaves in 2019, reported the FT.
Addressing MEPs in Strasbourg, Mr Barnier said his first job after "sufficient progress" is declared on the divorce issues will be to "fairly swiftly" hammer out out the final wording of a Withdrawal Agreement to put last week's deal into legally-binding form.
Meanwhile, the European Council "will also be looking at the definition of the transition period", including the question of the jurisdiction of the European Court of Justice, he said.
And, in an indication of further delay in the move to trade talks, he added: "There will be internal preparatory work that will need to be done with respect to our future relations with the UK."
Once trade is on the table, he made clear that the EU side will regard the integrity of the single market and the preservation of freedom of movement as "non-negotiable".
Mr Barnier praised Mrs May for her "courageous and respectable" conduct of negotiations, which required "trust" between the UK and EU. And he switched into English as he told MEPs: "Let's keep calm and continue."
But Ukip's former leader Nigel Farage denounced the PM as "Theresa the appeaser", telling MEPs she had "given in on virtually everything" during the "ritual humiliation" of her pre-dawn flight to Brussels to seal last Friday's deal.
Mr Farage said the proposed transition period was "the biggest deception yet played on the British people" and would mean the UK leaving the EU in name only, with the "very real possibility" that a future government will sign up to permanent membership of the single market and customs union.
Mr Barnier warned that upcoming talks will be "difficult, very tough, because the issues are extremely complicated and because the consequences of Brexit are very serious". He told MEPs the EU would have to be careful to ensure that future divergence from its regulations by the UK did not become "a tool for dumping".
In these talks, he said, "there are certain points which are non-negotiable - integrity of the internal market, the 'four freedoms' (of movement for goods, services, capital and people) which are absolutely essential and fundamental to the foundation of the internal market".
Mr Barnier said the EU and UK have the "shared objective to have good relations", but warned: "Obviously, we need to guarantee the integrity of the EU, which the UK has chosen to leave."
In an apparent reference to Mr Davis's comments, Mr Barnier told MEPs there could be "no going back" on the promises made in the joint report which were "prerequisites and conditions for continuing our work in a smooth fashion".
But he warned that "we are not at the end of the road" in terms of the divorce issues discussed in the first phase of talks, and revealed that the Irish border would continue to be the subject of a specific "negotiating path" to seek practical methods of meeting the ambition of both sides to keep the border open.
A European Parliament spokesman stressed that Wednesday's vote is not an indication of whether or not the European Parliament will give its consent to the final withdrawal deal, on which it holds a veto.