LONDON — As EU leaders are set to confirm “sufficient progress” in Brexit talks to move on to the next phase this week, Theresa May's team is shifting its focus to her next big problem: the gap.

With the Phase 1 deal almost in the bag and her Cabinet more or less falling into line, the U.K. prime minister goes into Thursday’s European Council and the Christmas break in better shape than many imagined possible.

The new year will bring a whole new set of challenges, the first of which is winning an early agreement on a two-year transition to ease the growing pressure from businesses. But that’s likely to be the easy part.

The real problem facing May is more profound: The money and assurances she has put on the table for an orderly divorce agreement don’t guarantee her a trade deal with the EU. The two issues are separate, each requiring their own treaty — the money and assurances come before Britain leaves the bloc. A trade deal would come after because the EU can only legally negotiate trade with countries outside the bloc.

Regardless of what her ministers might claim, the Brexit deal she agreed last week does not guarantee a comprehensive trade deal with the European Union. Legally, it cannot, as the EU’s chief negotiator Michel Barnier reiterated this week.

Come October 2018, by which time the EU wants the withdrawal treaty signed, the U.K. will have to agree to enter a transition period starting March 30, 2019, with nothing more than what Barnier calls “a political declaration” on what a trade deal treaty might — might — look like.

The Brits are categorical. Before next year is out, they want a real commitment on trade from the EU.

This interval between the two treaties — one on the divorce, the other on the future relationship — is deadly for the PM.

Phase 1 of the Brexit talks required her to meet the EU halfway — on money, the European Court of Justice and the Irish border question — to keep hope of a comprehensive post-Brexit trade deal alive. Phase 2 will require her to take a leap of faith that such an outcome will even be possible.

Once Britain has agreed to hand over the divorce bill, its leverage to ensure a good future trade agreement disappears, senior Conservatives said. “This is the key issue in the whole negotiation,” one senior former member of the previous government said, speaking on condition of anonymity. “The question for Britain is how to bridge the gap — how to somehow connect the two treaties. I’m amazed no one is talking about this.”

Inside the U.K. government, senior negotiators and officials involved in the Brexit negotiations recognize the problem, but believe there is a way.

“You can rest assured that no U.K. prime minister will sign a withdrawal agreement without having sufficient guarantees that the right arrangements will be in place at the end of the transition period for the start of the new relationship,” one senior U.K. official said, speaking on condition of anonymity.

The Brits are categorical. Before next year is out, they want a real commitment on trade from the EU.

Paragraph 96

Their problem is that they have already signed up to paying the £35 billion-£39 billion Brexit bill. That robs them of their key leverage for securing a trade agreement deeper than the EU’s bottom line offer: a Canada-style deal — or Canada Dry, as Brussels officials are fond of calling it.

For “dry” read: “no special financial services access.”

U.K. negotiators, however, believe their hand is strengthened by the final clause of Friday’s agreement. Paragraph 96 states that the joint agreement — including the U.K.’s entire financial obligation, and its pledges on the Irish border — is only agreed by the U.K. “on the condition” that the October 2018 withdrawal treaty “[takes] into account the framework for the future relationship.”

A great deal rests on this phrase. U.K. negotiators are exploring ways to ensure something similar to paragraph 96 forms part of the final exit treaty — committing the EU to negotiating a future trade deal before the end of the transition. Others believe that while the U.K. may have agreed to pay its divorce fees, it would be in a strong position to hold the money back if the EU reneged on its side of the bargain, even if this was only a political commitment.

May’s backbench Brexiteers have so far acquiesced to last week’s agreement — even if one prominent Brexiteer, former Conservative leader Iain Duncan Smith, admitted in the Daily Telegraph on Monday he was not “ecstatic” about it.

Increasingly, May’s trouble lies on her left flank, among Remainers in her party, whose strength is numerically small but potentially transformative when combined with Labour MPs against the government. The group showed their strength on Wednesday, by uniting to defeat the government for the first time on Brexit. Eleven Conservative MPs joined forces with Labour and the SNP on an amendment ensuring a final vote on the Brexit divorce treaty.

Earlier in the week, however, May was given an easy ride when she addressed the Commons about the phase one deal on Monday. Brexiteer MPs have generally gone quiet, placated by May’s insistence that day, drawing on paragraph 96, that “the offer is on the table” only so long as the U.K. and the EU agree on “the next stage and the partnership for the future.”

The U.K. does think it has some leverage left to persuade the EU to commit on trade earlier than is currently planned.

“If we do not agree that partnership, then the offer is off the table,” May said.

When Bernard Jenkin, another prominent Brexiteer, was asked whether his support for last week’s agreement was linked to the EU giving clear assurances on trade, he simply said: “As the prime minister indicated in the House of Commons on Monday.”

One senior MP in the Brexiteer ranks said on condition of anonymity that they wanted “early indication of a trade deal, otherwise we must crack on with WTO preparations” — in other words, no deal and no Brexit bill.

Extra leverage

The U.K. does think it has some leverage left to persuade the EU to commit on trade earlier than is currently planned.

It has been largely forgotten but there were four strands to the first phase of exit talks — Northern Ireland, citizens’ rights, money, and also “separation issues.” Talks on separation issues — which include the legal and regulatory status of U.K. and EU-produced goods already circulating within the single market — have been paused because “there is no point negotiating twice about these things,” a second senior U.K. official said.

“Separation issues” covers broad swaths of EU-U.K. cooperation, from recognizing the safety of car parts used and sold between each other’s manufacturers, to judicial cooperation and data protection law.

Essentially, both sides expect such issues will be dealt with in a future relationship agreement, the official said. If the EU was to renege on any “political” commitment to replace EU membership with a free-trade agreement, then both sides would face the dreaded “cliff edge” as all such separation issues would not have been tied up, the U.K. official said. It’s leverage of sorts, but the U.K. would be likely to be hardest hit.

If the EU were to “unpause” negotiations about separation issues this would be something of a red flag for trouble ahead — for the U.K.

Annabelle Dickson contributed reporting to this article.