LONDON — This much is clear: If there is a Brexit deal, the U.K. is going to pay for it.

Reports in the British press that London’s opening offer will be £36 billion (€40 billion) have focused attention on the all-important question of the U.K.’s financial obligations — one of the three exit issues on which the EU wants to see "sufficient progress" by October if Brexit talks are to move on to trade.

The specific figure of £36 billion is not recognized by Downing Street, Theresa May’s official spokesman said Monday.

That's no surprise, because while the EU wants an estimate of the bill, or at least a methodology, by October, U.K. government officials have long said they don't want to make a formal estimate until they have something to show for it. To do otherwise would be tantamount to paying tens of billions “for a conversation” about trade, said one official speaking on condition of anonymity.

The U.K. is clearly prepared to pay something. The longstanding government line has been that Britain is a country that meets is legal obligations.

An opening offer of £36 billion would be well below the minimum €65 billion (£59 billion) that the EU will ask for, according to the consensus among officials in Brussels.

The debate in Whitehall is not whether the U.K. should pay, but how that payment should be managed — and how it should be presented to Euroskeptic MPs and a British public that voted for Brexit at least in part based on the false promise of £350 million a week being repatriated from the EU budget, not 100 times that amount going the other way.

Staggered payment plan

The latest thinking is that part of the bill can be paid off as annual contributions to the EU budget, in exchange for market access on something close to existing terms, during a transition period.

The Telegraph, citing U.K. officials, reported that £10 billion a year net payments for three years would be proposed. Officials POLITICO spoke to said detailed proposals were not yet finalized, but the "principles seemed sensible."

Payments for three years after Brexit would end in 2021, by which time the EU's current budgetary round will have expired, and long-term planning for life without the U.K. could have taken place without any "cliff edge" drop in income.

Charles Grant, director of the Centre for European Reform think tank, said such a proposal had merit and could allow the Brexit talks to progress to the next phase.

“There is a strong feeling among U.K. and EU officials that the best way to present the Brexit bill is as payments during a transition, in return for market access,” he said. “Three years of paying about £10 billion a year would mean the U.K. handing over a large part of what the EU wants — not all of it, but enough to create goodwill and probably to get the EU to talk about trade.”

Brexit Secretary David Davis, speaking back in December, pointedly declined to rule out a Brexit deal that in one shape or other saw the U.K. paying for market access.

An opening offer of £36 billion would be well below the minimum €65 billion (£59 billion) that the EU will ask for, according to the consensus among officials in Brussels.

However, both sides’ projections of the Brexit bill are in part aimed at their hard-liners.

On the British side, these are the Brexit-supporting Conservative MPs, some of whom still insist that it’s the EU that should be paying Britain, not the other way round. They matter because, with the balance of power in parliament so precarious, there are enough of them to, in theory, sink the Brexit deal when it comes to a vote.

On the EU side, the hard-liners are those nation states that either receive the largest sums from the EU budget, or contribute the most. Both are determined that chief Brexit negotiator Michel Barnier’s strategy won’t leave them with a shortfall.

MP problem

Among Brexit-supporting Conservative MPs, the response to reports Britain is prepared to pay £36 billion were mixed.

Jacob Rees-Mogg, a darling of the party’s traditional right wing, said it was “much too much,” warning that the U.K. should not fall into the EU "trap" of conceding a bill early in negotiations. Peter Bone, one of the most hard-line Brexiteers, repeated a favorite argument that as net contributors for many years, Britain should be the one receiving a divorce bill.

However, a more emollient tone was struck by one significant figure. Suella Fernandes chairs the European Research Group group of Brexit-backing backbench Conservative MPs. Its former chair, Steve Baker, left in June after being given a job in the Department for Exiting the European Union. The group has played a pivotal role in mediating between MPs and the government on Brexit strategy, allowing ministers to ensure they were bringing the party with them.

Responding to the £36 billion reports Monday, Fernandes told POLITICO: “The U.K. has always paid its legal dues. This reported figure is however pure speculation. There’s no basis for it and we don’t recognize the figure. The government is focused on getting the best deal for the U.K.”

However, she added: “Any agreement must be fair to the EU and the U.K. taxpayer. A bad deal is worse than no deal.”

As recently as the spring, members of the European Research Group were arguing that May would never get a Brexit bill through parliament. After Davis’ comments in December, Baker said that paying for market access would not constitute free trade. Fernandes calling for a deal that is "fair" to the EU represents a concession — holding out hope that May will be able to convince the group to back a Brexit bill of some form.

The Boris factor

One possible obstacle remains in the way of Tory backbenchers assenting to a Brexit bill in the tens of billions: Boris Johnson.

The foreign secretary, who was the figurehead of the Vote Leave campaign, was closely associated with the £350 million a week promise, so his personal reputation is bound up with the issue. Last month, he invited EU negotiators to “go whistle” over their demands for a significant exit tab, positioning himself as a possible champion of those hard-liners who have yet to budge.

Legally, the U.K. could leave the EU without paying a penny if there is no deal, a fact that Brexiteers such as former minister David Jones have noted.

“This presents Boris Johnson and other hard-line ministers with a difficult dilemma: resign and perhaps fatally weaken the government; or accept a substantial budget payment and lose credibility with the Tory right-wingers," said Grant of the Centre for European Reform.

Johnson’s team declined to comment on Grant's suggestion that the foreign secretary might quit rather than accept a big Brexit bill — but the Brexit hard-liners, looking for a figurehead, will want a clear position from him as soon as MPs return from their summer holidays.

For some Brexiteers, however, the focus on what the U.K. owes risks the British side underestimating their negotiating hand.

David Campbell-Bannerman, a Tory MEP and ardent Brexiteer, said: “We should be going into these negotiations like any sensible negotiator would, being confident … We have a very strong position. The EU wants access to our market … [and] to the largest financial center in the world. Let’s not sell ourselves short on this.”