Brexit Secretary David Davis is heading for a Brussels showdown, with a demand for the European Commission to be more "flexible" in negotiations on the UK's withdrawal from the EU.
With the third round of formal talks beginning on Bank Holiday Monday in the Belgian capital, Mr Davis is pushing for EU negotiator Michel Barnier to be less rigid in his refusal to discuss the post-Brexit relationship between the UK and Europe.
Mr Barnier insists that progress must be made on key aspects of the withdrawal deal, including the UK's "divorce bill", ex-pat citizens' rights and the Irish border, before there is any talk of future arrangements for crucial issues such as trade.
Leaders of the 27 remaining EU states will decide in October whether the divorce talks have got far enough to move on to the second phase.
But Mr Davis is expected to urge Brussels to show more "imagination" to drive talks forward.
His Department for Exiting the EU (DExEU) has in the last week published a slew of papers setting out Britain's vision for a "deep and special" future partnership, dealing with issues ranging from the role of the European Court of Justice to customs arrangements and data protection.
Officials said the documents showed that conversations about the exit deal are inextricably linked with the shape of the future relationship.
And a UK Government source indicated that Mr Davis will warn Brussels not to "drag its feet" in discussions which must be completed in time for the deadline of March 2019.
"This round of negotiations will focus on thrashing out the technical detail on important matters related to us leaving the EU, and will act as a stepping stone to more substantial talks in September," said the source.
"The UK has been working diligently to inform the negotiations in the past weeks, and has published papers making clear our position on a wide range of issues from how we protect the safe flow of personal data, to the circumstances around Ireland and Northern Ireland.
"Now, both sides must be flexible and willing to compromise when it comes to solving areas where we disagree.
"As the EU itself has said, the clock is ticking so neither side should drag its feet."
Labour's shadow Brexit secretary Sir Keir Starmer said: "After two faltering rounds of negotiations, there is still a startling lack of clarity on the Government's Brexit priorities.
"David Davis is calling for more 'flexibility' and 'imagination' from Brussels but is yet to put forward a clear and serious offer on any of the key negotiating issues. The Government's incoherent and inadequate position papers on Brexit have left more questions than answers and show the Government remains divided and unprepared for the huge task ahead.
"Instead of 'constructive ambiguity', we need a government that can deliver a strong Brexit deal. That means committing now to transitional arrangements and reaching a long overdue agreement on key issues such as citizens' rights."