LONDON — A new school term of Brexit talks has begun, but there’s only one exam question that matters: the Irish border.

A handful of tricky issues remain in the 20 percent of the withdrawal deal text yet to be agreed between London and Brussels — the EU's luxury food names and the mechanism for settling future disputes, to name two — but at this late state, it is highly unlikely anything other than the Irish question could break the U.K.-EU withdrawal negotiation.

Officials on both sides say that a summer of ongoing talks has ended with zero progress on the critical element of the Ireland issue — the so-called backstop arrangement for avoiding a hard border no matter what Brexit deal emerges.

Brussels has been saying for months that without it, there can be no deal. It's a message that chief negotiator Michel Barnier reinforced on Friday following the latest round of talks with his opposite number Dominic Raab. "This backstop is critical to conclude the negotiations because ... without a backstop, there is no agreement," he told journalists.

But while both sides have been keen to emphasize the progress that has been made on the Withdrawal Agreement, officials familiar with the talks are more candid about how little has been achieved on the Ireland issue since Theresa May won the backing of her Cabinet at her Chequers country residence in July for her negotiating plan.

Failure to solve the Irish question would render the debate on the future relationship academic.

“It’s as we were,” said one senior U.K. official when asked what the summer’s talks had yielded on Ireland. An EU diplomat on the other side of the negotiation agreed, adding wearily: “We are stuck on the old issues. There’s nothing new. It’s all the same thing.”

A second EU27 official familiar with discussions agreed: “That’s it in a nutshell.”

The fate of the Chequers plan — which is coming under sustained bombardment from Brexiteers in May's own party — and the talks with Barnier on the future relationship matter hugely. But failure to solve the Irish question would render the debate on the future relationship academic.

Two backstops

Ireland and the future are, though, inextricably linked.

Two backstop proposals are currently on the table. The EU’s says that if the final future relationship deal (to be signed sometime after the U.K.’s March 2019 exit date) does not remove the need for customs and other checks at the EU’s borders with the U.K., then Northern Ireland should effectively remain in a customs union with the EU.

Theresa May says this would create an intolerable economic frontier within her country, and has put forward an alternative. But this U.K. proposal is predicated on the EU accepting the central pillar of the Chequers plan for the future relationship — a common rule book on goods and agri-food that would remove the need for regulatory checks at the north-south border. At the same time, the plan would allow the U.K. to diverge from EU rules on services.

But in an interview with the Frankfurter Allgemeine Zeitung newspaper, which took U.K. officials by surprise, Barnier this weekend gave his firmest rejection of that proposal, saying that allowing the U.K. to “pick pieces” of the single market would be the beginning of the end of the European project.

His language echoed that which EU diplomats have been using privately all summer. “I’d say to the British: You do not understand what you are asking of the [EU]27,” said the first EU diplomat. “You are asking to start the process that will dissolve the [European] Union.”

No wonder then that on the EU side the expected outcome of the backstop negotiation is that the U.K. will have to swallow a version of the EU’s proposal.

Barnier talks about “de-dramatizing” the backstop — and diplomats are conscious that “rebranding” could be needed. Diplomats close to the negotiation note that trade between Northern Ireland and the rest of the U.K. is already subject to some checks, for instance on live animal imports. A minor uptick in this kind of activity could be dressed up as something short of a “border in the Irish Sea,” they argue.

“There could be some rebranding, repackaging,” the first EU diplomat said, before joking: “Not ‘customs patrols’ but ‘friendship patrols.’ That kind of thing.”

Salzburg summit

The next major Brexit staging post will be the informal meeting of EU leaders in Salzburg on September 20. U.K. officials want an opportunity for May to make her Chequers pitch, and then for the EU27 leaders to carve out time for a Brexit discussion — although they are not presenting it as a make-or-break moment.

According to one senior U.K. official, the British negotiating team will be looking for a formal acknowledgement that Chequers and the subsequent white paper represent an “evolution” of the U.K.’s Brexit position. This language is important as European Council guidelines from June reiterate that “if the UK positions were to evolve, the Union will be prepared to reconsider its offer.”

The Brits may get their wish. But not much more.

“We recognize that Theresa May made a huge step with the white paper,” said the first EU diplomat. “We can assume there will be some discussion of Brexit at Salzburg. But it would be wrong to assume it will be groundbreaking, or that there will be any concessions.”

“There is a feeling in Brussels that given there are party conferences in the U.K. between now and then, we would require a bit more time after October Council" — EU diplomat

After Salzburg, the next diary day is October 10. Give or take a few days, this is the final date, some EU diplomats believe, by which a Brexit deal would need to be done in order to go through the necessary Brussels and member country bureaucratic processes in time to be signed off by leaders at the October 18 European Council summit — still, officially at least, both sides’ goal.

This is becoming more and more unrealistic and Barnier has acknowledged publicly that the deadline could slip to November. “There is a feeling in Brussels that given there are party conferences in the U.K. between now and then, we would require a bit more time after October Council,” a third EU diplomat said.

The first diplomat said a specially arranged November summit, at which leaders would either be asked to resolve one or two outstanding negotiation issues at political level, or be presented with a finalized deal to sign off on, is a possibility, but that it is “too early to talk about choreography.”

The political declaration

The final unknown, assuming the two sides can resolve the Irish backstop, is the political declaration on the future relationship — the complementary document to the formal Withdrawal Agreement. It is on this, more than the details of the withdrawal treaty itself, that the U.K. parliament is likely to judge May’s Brexit deal.

A detailed political declaration based on the Chequers proposal (unlikely as this seems given the EU opposition) or a Norway-like alternative, would almost certainly be rejected by May’s Brexiteer backbenchers, led by Boris Johnson, who on Monday decried Chequers as “surrender.”

But a political declaration detailed enough to make clear a change of course toward a Canada-like free-trade deal risks being rejected by Remain-leaning Conservative MPs. The opposition Labour Party is likely to vote against any deal May brings back.

Publicly, the U.K. still insists it wants the political declaration to be detailed and precise. Brexit Secretary Raab told the House of Lords EU committee last week that his goal would be “avoiding fudge and having some clarity.”

But how he and May will first square the Northern Ireland circle to even reach this stage, and then deliver a vision of the post-Brexit future their party can get behind, is a question no one in London or Brussels can yet answer.