By Kylie MacLellan and Thomas Escritt
UNITED NATIONS/BERLIN (Reuters) - The European Union's Brexit negotiator said on Monday it was difficult to see a way to break the Brexit impasse as British Prime Minister Boris Johnson's demand to drop an insurance policy for the Irish border was unacceptable.
More than three years after the United Kingdom voted by 52%-48% in a referendum to leave the EU, Brexit's future remains uncertain, with options ranging from a turbulent no-deal exit to abandoning the entire endeavor.
Hopes of a deal to ease the transition were stoked when Johnson said the shape of an accord was emerging and European Commission President Juncker said an agreement was possible.
But EU negotiator Michel Barnier cast doubt on the likelihood of a deal and reaffirmed that the bloc could not agree to London's demand to remove the Irish "backstop", a policy to prevent a return of border controls on the island of Ireland, without a serious alternative.
"I am sure you understand this is unacceptable," Barnier said during a news conference in Berlin alongside German Foreign Minister Heiko Maas.
EU sources said no proper alternative for the border between Northern Ireland, a British province, and Ireland that ensures the integrity of the EU single market and customs union has been proposed yet by London, so there is no prospect of an immediate breakthrough.
In a quip about British talk of virtual checks on the border, Barnier said: "I don't know how to inspect a cow with virtual methods."
"Based on current UK thinking, it is difficult to see how we can arrive at a legally operative solution which fulfils all the objectives of the backstop," he said. "It is in a very sensitive and difficult phase."
Following a meeting with Johnson on the sidelines of the United Nations General Assembly in New York, European Council President Donald Tusk wrote on Twitter: "No breakthrough. No breakdown. No time to lose."
Johnson has vowed to take Britain out of the EU by the current deadline of Oct. 31, with or without a transition deal.
With the outcome of the marathon Brexit process still mired in doubt, the United Kingdom Supreme Court will rule on Tuesday whether Johnson's Aug. 28 decision to suspend parliament was unlawful.
BREXIT IN PLAY?
If its 11 justices rule against the government at 10:30 a.m. (0930 GMT) on Tuesday, Johnson could be forced to recall parliament - a step that widens the scope for lawmakers to block his "do-or-die" Brexit plans, which they fear could see Britain crashing out of the EU without a deal.
The opposition Labour Party will vote to decide its strategy on Monday, with leader Jeremy Corbyn heading for a showdown with the widely pro-EU party membership over whether Labour should endorse a policy of remaining in the bloc.
Ireland is crucial to any Brexit solution. Unless the Irish border backstop is removed or amended, Johnson will not be able to win parliamentary approval - but Ireland and the EU are loath to sign a deal without a solution to the border.
Johnson has said he wants to secure an amended deal at an EU summit on Oct. 17-18, and that "a large number of the important players", including Britain, Germany, France and Ireland, are keen to reach an agreement.
But he says that means a removal of the backstop, which aims to avoid the reimposition of checks along what would be the sole British land border with the EU by having London follow EU rules on trade, state aid, labor and environmental standards.
He said it was important the United Kingdom "whole and entire" was able to break away from EU law in future.
"The problem with ... the current backstop is that it would prevent the UK from diverging over a huge range of industrial standards and others," he said. "We may want to regulate differently but clearly there is also a strong incentive to keep goods moving fluidly, and we think we can do both."
A UK government official said Britain had put forward detailed proposals and had "made a move to try to get this process moving along".
"What we are looking to see from the EU is that they are prepared to engage seriously with these ideas," the official said.
An EU diplomat said on Monday that Johnson's government was sticking to a counter-productive approach imposing "its own, self-made problems on us and demand(ing) that we fix them.
"... Why would we bend our rules for a country that is leaving? Why put ourselves at risk for a third country? Why help smooth their departure at our own cost?" the diplomat said.
(Additional reporting by Michael Holden in London, Elizabeth Piper and William James in Brighton, England, Thomas Escritt and Paul Carrel in Berlin and Gabriela Baczynska in Brussels; Writing by Guy Faulconbridge; Editing by Mark Heinrich and Jonathan Oatis)