EU's Chief Brexit Negotiator Michel Barnier | Charles McQuillan/Getty Images Brussels slams UK’s Brexit ‘renegotiation’ attempt EU officials view the UK’s customs ‘backstop’ plan as an attempt to renegotiate the transition period.

The U.K.'s latest proposal to buy yet more time before a post-Brexit customs regime can be put in place is dead-on-arrival in Brussels, EU officials and diplomats say.

The plan is billed by London as an alternate "backstop" to prevent the re-creation of a hard border on the island of Ireland while it prepares a more permanent customs solution, but the EU sees it as requiring a whole new round of complex negotiations for a "bespoke" transition lasting beyond December 2020. That would amount to something Brussels is unwilling to do — namely to reopen talks on the already agreed transition period under which the U.K. will remain subject to EU law.

Prime Minister Theresa May won the agreement of her Brexit war Cabinet for a time-limited post-Brexit customs arrangement to act as a stop-gap solution two weeks ago. Its rejection in Brussels — even before her officials have formally presented it in the talks — shows the two sides on a direct collision course just one month before a European Council summit that has been set up as a key milestone in the talks.

Without significant progress on the Irish border question by then, there are doubts that there will be enough time to secure an overall withdrawal agreement by the October deadline.

For Brussels and the EU27, London's agreement in March on a "standstill" or "stand-in-place" transition is viewed as a rare display of common sense from the British government on Brexit.

Despite the objections of many hard-line Brexiteers, who were enraged at the idea of the U.K. remaining subject to EU laws and rules while losing its vote at the European Council, May and her top negotiator, David Davis, acknowledged that there is no time to negotiate a whole new arrangement that would simply expire upon adoption of a broader deal on the "future relationship."

Since then, however, a dispute has only deepened over the EU's "backstop," which would keep Northern Ireland in the EU's customs union until a permanent solution is reached for the Irish border.

Adding to the pressure on London, the EU's chief negotiator, Michel Barnier, referred in a speech on May 26 to British delays as "a game of hide-and-seek."

For their part, U.K. officials say Brussels is obstinately refusing to accept that the EU version of a backstop is off the table. There is widespread political agreement in London and Belfast that it violates the constitutional integrity of the U.K.

Complicating matters further, May and her ministers have been advised by her customs authority that neither of the post-Brexit customs options under discussion in London could be ready in time for the end of the transition period (or implementation period, as the U.K. government calls it) on December 31, 2020.

"The fact that it's time-limited is the main irritant" — EU diplomat

May's official spokesperson reacted angrily last Tuesday to what he referred to as "soundbites and negative anonymous briefings" from Brussels about the U.K.'s proposals. “It is imperative that we keep these talks constructive," he said, apparently referring to a technical briefing in Brussels from a senior EU official.

But EU officials and diplomats say they have already heard enough to know that the U.K.'s plan is unacceptable — mainly because the U.K. government has stated repeatedly that it would be "time-limited."

Because there is no guarantee of when negotiations on the future relationship will be concluded — or even if those talks will be successful — EU officials say any proposed backstop plan cannot have an expiration date.

"The fact that it's time-limited is the main irritant," said one EU diplomat.

Equally problematic for Brussels is that the U.K.'s plan would require complex negotiations on a "bespoke" transition period — including how much Britain would contribute to the EU's budget.

"It would be a very, very complicated thing to negotiate," one senior EU official said. "I really don’t see how you get there."

At the technical briefing that drew Westminster's ire, a senior EU official accused the U.K. of "chasing a fantasy" with its proposal to stay within the EU's external tariff for a limited period of time.

What received less attention, however, was the official's flat, plain-spoken rejection of the U.K. proposal on two technical grounds: that the backstop providing "regulatory alignment" must apply only to Northern Ireland and that it cannot be time-limited.

"It has to be Northern Ireland specific — let me make it very clear," the official said at the technical briefing. "The regulatory alignment option is not available on an all-U.K. basis because it would amount to selective participation in the single market. And we would need to understand the rules in the backstop for the customs dimension, which cannot be time-limited."

But even if the U.K.'s proposal were tailored narrowly to Northern Ireland — a political no-go for May's parliamentary allies in the Democratic Unionist Party — officials said Brussels would oppose it because it would effectively create the need for a "bespoke" transition.

"These other ideas for a transition period No. 2 after the one already agreed — this is not realistic," said the first senior EU official. "It would indeed raise a very big number of questions. It's difficult enough to agree on a future relationship."

Nicola Sturgeon, the first minister of Scotland, who favors the U.K. permanently remaining in the EU single market and customs union, said last Monday that ultimately Westminster would have to make a choice between its conflicting positions.

"Reality at some point has to bite for the U.K.," Sturgeon said. "Currently, the government is trying to reconcile a whole plethora of irreconcilable issues."

Annabelle Dickson contributed reporting.