LONDON — While Theresa May and her Cabinet fight it out in the latest battle over Britain's Brexit position, officials from both sides of the channel are convinced she has already run out of time.

On Thursday, the day after May failed to convince her most senior ministers to back her position on Britain's future customs arrangements, Brexit Secretary David Davis pushed for a second meeting of the so-called war Cabinet in a bid to break the deadlock.

Ministers have told British officials to work up emergency compromise proposals by “tweaking” the two current options on the table in a bid to find consensus, one senior U.K. official familiar with the discussions said.

But even if May's top team can reach consensus — and Brussels drops its current opposition to the U.K.'s plans as they stand — there is almost zero chance either of the two options will be ready by the end of the proposed transition period on December 31, 2020, according to senior U.K. and EU officials.

Even if May can clear the political obstacles that still stand in the way of a deal, she will need more time to implement the technology and processes needed for any new customs system.

The U.K. will therefore have to negotiate an extra transition period on customs — or an interim arrangement of some kind — to bridge the gap between the end of the transition and the point at which the new customs arrangement is ready, the officials said.

“The Cabinet have two decisions to make,” one senior Brussels-based official said. “First, where they want to go, but then, second, how to get there.”

As the political storm around the U.K. prime minister intensifies, such practical constraints also become more apparent, further threatening her fragile grip on power.

In Brussels, diplomats attribute the delay to Thursday's local elections in England.

"We haven't had any Brexit discussions on substance since March, nothing is really advancing so [there's] nothing to discuss. Probably there will be some movement after U.K. local elections," a senior EU diplomat said.

The customs arrangement favored by May is now firmly out of contention after Wednesday’s meeting of her top team.

Back in Westminster, however, and despite her party bracing for a bad night, the local elections looked like the least of May's troubles.

Brexit bridge

Officials involved in the talks say the U.K. prime minister has days rather than weeks to find a customs compromise in order to make enough progress to satisfy EU negotiators ahead of June’s European Council summit.

The U.K. is adamant June is an “arbitrary” deadline it does not recognize, insisting October is the real timeframe. In private, one senior EU official admitted June was only a “staging post.” However, they said Brussels is determined to make some progress soon if a final withdrawal agreement is to be agreed by October.

However, three senior U.K. and EU officials in Brussels and London said that even if May can clear the political obstacles that still stand in the way of a deal, she will need more time to implement the technology and processes needed for any new customs system. One official said a "four- or five-year" extension of the transition period for customs might be necessary, but only if the U.K. comes up with an "operational" plan.

The prospect of de facto extending membership of the customs union while new arrangements are put in place is likely to spark fresh recriminations for the prime minister in parliament. Brexiteer MPs — and in particular their figurehead Jacob Rees-Mogg, chair of the European Research Group — only reluctantly swallowed a transition arrangement that is scheduled to end in December 2020. Rees-Mogg considers the terms of transition — subject to the EU’s rules, but with no say in them — as “vassal state” status and would likely resist any extension.

To complicate things further, Davis has repeatedly insisted that extending the agreed transition period will not be necessary.

“I do not expect the solution to be an extension of the customs union,” Davis told Rees-Mogg during a session of the Commons Brexit select committee. “I would view that on my part as a failure.”

Appearing before MPs in the House of Commons Thursday, Davis played down the pressure on the government to make up its mind between the two customs proposals on the table.

“Both of these approaches have merits and virtues, both have some drawbacks and that’s why we’re taking our time over the discussion on this,” he said.

However, it is now accepted, two senior government figures said, that the customs arrangement favored by May is firmly out of contention after Wednesday’s meeting of her top team. The so-called hybrid partnership plan, which would see the U.K. continue to collect tariffs on goods destined for the EU, had been the favored option at the top of government, officials said, preferred because May's advisers felt it was potentially sellable to Remain-leaning Conservative rebel MPs who still want the U.K. to stay in a customs union.

Davis repeated in the Commons Thursday that the government is in agreement on the need to leave the customs union. Asked how certain he was that would happen, given the widespread support in the House of Commons for continued membership, Davis said: “Will 100 percent do?”

A third way is needed — and fast. But as Davis told MPs, London still insists it won’t be rushed.

“It’s frankly incredibly important that we get this right, not just for trade but for the extremely sensitive issue of maintaining the peace process in Northern Ireland. I don’t undertake to put an artificial deadline on something as important as that.”

Jacopo Barigazzi contributed reporting.