Britain's Brexit minister David Davis caused alarm by saying that a crucial agreement struck last Friday on separation arrangements was a "statement of intent" rather than "legally enforceable".
Chief EU negotiator Michel Barnier said that "all our points of agreement are now closed" under the deal made by Prime Minister Theresa May, which paves the way for an EU summit this Friday to open trade talks.
"We will be vigilant. We will not accept any backtracking from the UK," Barnier said at a press conference.
Asked about Davis's suggestion that a trade deal could be agreed in coming months to take effect a minute after Britain leaves the EU, Barnier said it was "not possible", adding "and David Davis knows that full well".
EU President Donald Tusk said in a letter to leaders ahead of the summit that there was now a "furious race against time" to reach a deal on future relations.
The European Parliament's Brexit pointman Guy Verhofstadt said Davis's comments at the weekend were "unacceptable" and would lead to a "hardening of the position" of the EU.
"It's clear that the European Council will be more strict now in saying... we want that these commitments are translated into legal texts before we make progress in the second phase," Verhofstadt said, referring to the summit.
The European Parliament will now vote on a resolution on Wednesday which backs the Brexit deal - but which unusually mentions Davis by name, saying his comments "risk to undermine the good faith that has been built during the negotiations."
