The European Union's chief Brexit negotiator Michel Barnier said Monday that London's current position did not offer any "basis to find an agreemen" on leaving the bloc.
Britain is racing toward its October 31 departure without an exit agreement and faces economic disruption that the government admits could cause food shortages and spark civil unrest.
"Based on current UK thinking, it's difficult to see how we can arrive at a legally operative solution which fulfills all the objectives of the backstop," said Barnier in Berlin after talks with German Foreign Minister Heiko Maas.
"For the moment, we don't have the basis to find an accord," he added.
The "backstop" is intended to guarantee that there is no hard border between EU member Ireland and the UK's province of Northern Ireland and to protect the EU's single market.
Barnier's downbeat assessment of the state of negotiations between London and Brussels came after his meeting last week with his British counterpart Stephen Barclay.
Talks will continue this week in Brussels and New York, with meetings planned between British Prime Minister Boris Johnson and the EU's President Donald Tusk later Monday.
"We remain open to talks, and to progress and to listen (to) the new ideas from the UK also regarding the future relationship," Barnier told journalists.
He stressed however that it was up to London to find a solution to the deadlock.
"The ball is in the court of the British," he said.
Barclay, speaking in Prague, insisted however that London remained committed to a deal with the EU, but said this could not include a backstop because that would make it unacceptable to the British parliament.
"The backstop would require the UK to accept laws over which citizens would have no say," he added.
The EU has warned against a disorderly exit, with the European auto industry becoming the latest on Monday to sound the alarm, bracing for a potential "seismic" impact on car-making.