Agreement on a transition period to smooth Britain's exit from the EU is "not a given", Brussels' chief Brexit negotiator has warned.
After the latest round of talks between officials in Brussels, Michel Barnier said that three "substantial" disagreements remain over plans for the transition, expected to last around two years after the date of withdrawal in March 2019.
Declaring himself "surprised" by the UK stance, Mr Barnier said that Britain had to accept the "ineluctable consequences" of its own decision to quit the EU.
And he said: "To be quite frank, if these disagreements persist, the transition is not a given... Time is short - very short - and we haven't a minute to lose if we want to succeed."
Mr Barnier also warned that checks on the border between Northern Ireland and the Republic will be "unavoidable" under Theresa May's Brexit plans for the UK to leave the single market and customs union.
He confirmed that Brussels is now drawing up a legal definition of the regulatory alignment that must continue between Northern Ireland and the EU in order to avoid the creation of a hard border.
London had so far provided no proposals for the kind of technological fix which could get round this, he told a press conference in the Belgian capital.
Mr Barnier's comments came after the EU's publication of a position paper on the proposed transition deal provoked fury from Brexit Secretary David Davis, who branded the document "discourteous" and "unwise".
In a sign of how the atmosphere has soured, Mr Davis accused the EU of not acting "in good faith" over the inclusion of plans for sanctions if UK fails to keep to the withdrawal agreement.
But Mr Barnier said he could not understand the "uproar" in London over the position paper.
"There is no wish whatsoever to punish," he said. "We are trying to draft an international agreement on a solid basis that can be effectively implemented."
He pointedly noted that he had been expecting an update on Friday morning from the UK Government on its vision for the future relationship, but that the meeting had to be cancelled because of diary difficulties on the British side.
At his meetings with Mr Davis in London and Brussels this week, it was clear that three "substantial" points of difference remain between the two sides' visions of how the proposed transition period would work, said Mr Barnier.
- The UK's refusal to guarantee permanent rights to EU nationals who come to live and work in Britain during the transition period;
- The UK's demand for a right to object to the application of any new EU rules and laws introduced during the transition period;
- Britain's wish to continue participating in the development of new policies in the area of justice and home affairs during the transition.
Disagreement also remained over references to European Court of Justice jurisdiction in the withdrawal agreement, which Brussels regards as essential for the deal to be "credible and sustainable", he said.
Mr Barnier said: "To be frank, I am surprised by these disagreements. The positions of the EU are very logical, I think.
"The UK wants to enjoy the advantages of the single market, the customs union and common policies. It has therefore to accept all the rules and the obligations until the end of the transition. That's very logical.
"It also has to accept the ineluctable consequences of its decision to leave the EU, to leave its institutions and its policies."
The UK Government has issued a technical note calling on the EU and more than 100 countries with which it has international deals to treat the UK as if it were a member state so signed agreements on the likes of trade, nuclear co-operation and aviation can continue to function during the transition.
Michel Barnier said: "I have read that and I think that confirms our legal assessment, our legal analysis when it comes to international agreements."