The EU’s chief negotiator Michel Barnier has warned that “Brexit is not a game” after being asked whose court he thought “the ball is in”.
He was speaking to the BBC after a “good” and “constructive” working lunch with the UK’s Brexit Secretary David Davis as negotiations continue.
Next week EU leaders decide if enough progress has been made to move talks on to the UK’s future trade relations.
The UK is set to leave the European Union at the end of March 2019.
Both EU and UK teams have said the ball is in the other side’s court this week – implying that it is the other side that has to make the next concession.
This week’s talks came as ministers sought to ease disquiet among Brexit-backing MPs about the UK’s strategy for a two year “transition” period between being a full EU member and the UK’s eventual post-Brexit relations with the EU.
Downing Street said it wanted the process to be “as smooth as possible”.
After Theresa May briefed her cabinet on Tuesday, a No 10 spokesman said the government hoped to negotiate a deal with the EU on the terms of exit but was prepared for all eventualities – a reference to what some believe is the growing likelihood of a “no deal” scenario.
On Monday the PM told MPs “real and tangible” progress had been made in negotiations so far.
But the EU has signalled it is not prepared to agree to move on from the initial phase of talks, covering the financial settlement, Northern Ireland and citizens’ rights.
The final decision on this will be taken at a European Council summit on 19 October, with the UK keen to move on to discussing what its future relationship with the EU will look like.
Taking questions from MPs on Monday, Mrs May also confirmed that the UK could remain subject to the rulings of the European Court of Justice during a planned two-year transition period after Britain leaves the EU in March 2019.
This was criticised by some pro-Leave campaigners, with backbencher Jacob Rees-Mogg telling the BBC he was “troubled” by the PM’s statement.
“If we’re remaining under the jurisdiction of the ECJ then we haven’t left the European Union or the date of departure is being delayed,” he said.
Foreign Secretary Boris Johnson and Environment Secretary Michael Gove – two key figures in last year’s Leave campaign – both issued statements backing Mrs May’s comments.
BBC assistant political editor Norman Smith said their intervention underlined the government’s nervousness about the reaction of Tory Brexiteers over the European Court of Justice’s jurisdiction – which for many of them is a “red line”.
Mr Johnson, who has been accused of undermining the PM with his recent interventions on Brexit strategy, issued a statement backing her “powerful vision”.
On Twitter, Environment Secretary Mr Gove said: “Strong statement from PM on Brexit – let’s be pragmatic over implementation to secure maximum freedom to diverge from EU in end state.”
Asked on Tuesday about the role of the ECJ during a transitional phase, No 10 said business should only have to adjust to one set of changes following the UK’s departure.
But it reiterated the PM’s hope that a new dispute resolution system could be devised as quickly as possible to assume the ECJ’s functions and settle the matter once and for all.