LONDON — Forget the imminent negotiations in Brussels — it’s the Brexit debate taking place in London that looks set to shape the U.K.’s future.

Less than a week after a shock election result robbed Theresa May of her parliamentary majority, forces both in and outside of government have mobilized to try and exploit her political weakness, and change the course of the U.K.’s departure.

Opposition to May’s Brexit strategy — which until last week appeared to be set in stone — is crystallizing around one main goal: keeping the U.K. inside the EU’s tariff-free customs union.

May’s plan for the U.K. to strike post-Brexit free-trade deals around the world implies an exit from the key tenets of the EU’s customs union, which block such freedoms. But British Chancellor Philip Hammond is pushing for a rethink at the top of government to limit the potential damage that tariff barriers would have on EU trade and the economy.

His position is supported by a group of backbench soft Brexit Conservative MPs who now see their chance, in a reshaped Parliament, to find common cause with Labour’s bolstered ranks and alter the government’s stance.

Staying in the customs union, but potentially still outside the single market, would take the U.K. close to “the Turkey model” of EU involvement. While it would spare some of the economic turbulence of a cliff-edge exit from the EU, it could also dash the hard Brexiteers’ hope of creating an unrestricted, free-trading “global Britain.”

Senior officials familiar with Labour's stance are clear that Corbyn and his shadow cabinet see an opportunity to alter the direction of Brexit.

“It’s perfectly feasible,” said Anand Menon, professor of European politics and foreign affairs at Kings College, London. “That’s where Turkey is, with a bespoke deal with the European Union and the ability to agree some free-trade agreements.”

“But one of the implications of being in the customs union would be that [International Trade Secretary] Liam Fox doesn’t have a job.”

No coming back

While the coming clash over the customs union could reshape Brexit, one thing all players in all parties are agreed on is that Brexit is still on. Warm words from Emmanuel Macron and the German Finance Minister Wolfgang Schauble, suggesting that the door to the U.K. remaining in the EU is still open, caused barely a ripple in Westminster.

Even Ken Clarke, the former Conservative chancellor and arch-Europhile was clear on BBC Radio 4’s Today program Wednesday. Macron is “wrong,” he said. “We are destined now to leave the European Union. But there’s not been a sensible debate so far on the basis on which we leave.”

The question of single market membership also appears to have been settled — as that would imply freedom of movement and recognizing the jurisdiction of the European Court of Justice. Soft Brexit Conservative MPs, speaking on condition of anonymity, emphasized the importance of the customs union question, while Labour officials signaled that the party would not steer away from its manifesto commitment that freedom of movement — a prerequisite of single market membership — will end.

Hammond — who will outline his position in more detail during his annual Mansion House speech to City of London grandees Thursday — is finding common cause with Scottish Conservative leader Ruth Davidson, in pushing the government in a different direction on other Brexit questions, including immigration, the transition, and chiefly, the customs union.

Davidson, who led the Tories to their best general election performance in Scotland since 1983, and now has a bloc of 13 Tory MPs under her sway, wasted no time after the election in calling for an “open Brexit” that put “free trade and economic advancement at the heart of a Brexit deal.”

Hammond meanwhile is marshaling City of London support for a soft Brexit — specifically a change of heart on membership in the customs union. That would mean a rethink of the government’s decision to plow ahead with an independent trade policy outside of the EU: a stance that would put him on a collision course with Fox, whose job description is to deliver free-trade deals around the world.

Labour’s position

Labour will set out its post-election Brexit position later this week. Senior officials familiar with the party’s stance are clear that Corbyn and his shadow cabinet see an opportunity to alter the direction of Brexit. One official pointed out that Labour’s manifesto does not rule out staying in the customs union.

“We’ve never ruled out the customs union, as May had done,” the senior official said. “The option should remain on the table.” Labour is not wedded to customs union membership, but open to it, and the issue creates less of a political dilemma than single market membership, which would likely mean continued freedom of movement, which the official said will “end” — as per the party’s manifesto.

“The government needs to change their position,” the official added. “We’re not going to change ours.”

That stance makes possible an agreement with Conservative soft Brexiteers.

May still needs to get her Brexit deal through parliament at the end of the process. Without a clear majority, she’s vulnerable to the ex-Remainers in her party finding common cause with Labour’s swelled ranks and getting the numbers to reject it — an outcome that would almost certainly topple her government. Labour could even use the Great Repeal Bill legislative process to force a vote on the Brexit strategy early on.

However, Conservative MPs, even the most ardently pro-EU, will not want to risk anything that could too seriously destabilize the government — for fear of strengthening Labour and Jeremy Corbyn. They may hope that, to avoid losing any votes on Brexit, the prime minister softens her position sooner rather than later. Whatever happens, weeks of political horse-trading lie ahead.

As one minister put it, with the patterns of voting so unpredictable, and May’s minority position, MPs are empowered again, and we’re back to “politics in the raw.”

“With the election, we’ve just had a very major change in circumstance, and I think we need to consider very carefully what that actually means,” said one senior pro-EU Conservative MP.

Another said that “the make-up of the new House of Commons does put some Brexit options back on the table,” singling out membership of the customs union.

Hard Brexit resistance

May will, however, have arguments on her side if she decides to stay the course and push through her hard Brexit strategy.

“If you watched her debate appearances during the course of the election, she got the biggest cheers and the most positive response when she talked about her plans for Brexit and how she was going about it,” said Matthew Elliot, the former chief executive of the Vote Leave campaign, and a senior fellow at the Legatum Institute think tank.

“They will obviously try, and they are trying, but I don’t think it will get any traction" — Matthew Elliot, former chief executive of the Vote Leave campaign

He said efforts to create a “soft Brexit” will struggle to get off the ground.

“They will obviously try, and they are trying, but I don’t think it will get any traction, not least because the government’s negotiating position is already set in stone with the white paper and obviously was included in the manifesto,” he said.

“The EU has already taken this into account when coming up with their negotiating position, so I think very quickly the debate will move to the negotiations between the British government and the rest of the EU,” he added. “Any internal debate within the U.K. will be seen as a bit of a sideshow.”

Philip Hammond, the Labour Party, and a small but newly-powerful group of Conservative MPs, have other ideas.