The EU on Tuesday rejected British Prime Minister Boris Johnson's demand to scrap the Irish border backstop plan to achieve a Brexit deal, saying he had offered no workable alternative.
Johnson wrote to EU Council President Donald Tusk on Monday to insist that Britain could not accept what he called the "anti-democratic" backstop, a mechanism to avoid border checks between EU-member Ireland and British-ruled Northern Ireland.
Since taking office last month, Johnson has been adamant Britain will leave the European Union on October 31 come what may and has stepped up preparations for a chaotic "no deal" departure that would cause major economic disruption.
But the European Commission, the EU executive which has led Brexit negotiations with London, dismissed the proposal in Johnson's letter that the backstop could be replaced with a "commitment" to find "alternative arrangements".
"The letter does not provide a legal operational solution to prevent the return of a hard border on the island of Ireland," commission spokeswoman Natasha Bertaud told reporters.
"It does not set out what any alternative arrangements could be, and in fact it recognises there is no guarantee that such arrangements will be in place by the end of the transitional period."
The clash comes as Johnson prepares to travel to Berlin and Paris, where he hopes to convince German Chancellor Angela Merkel and French President Emmanuel Macron to give ground in the search for a Brexit deal.
Telephone talks of nearly an hour on Monday evening with Irish Prime Minister Leo Varadkar failed to yield any obvious breakthrough, with Varadkar's office issuing a statement afterwards insisting the withdrawal agreement containing the backstop cannot be changed.
Johnson's diplomatic offensive continues at a G7 summit in the French town of Biarritz at the weekend, where Johnson hopes to show off his warm relations with US President Donald Trump as a signal of post-Brexit Britain's global ambitions.