LONDON — Meet the real leader of the opposition.

Following Thursday’s bombshell High Court decision that forces Theresa May to get parliamentary approval before taking Britain out of the EU, Labour’s Brexit spokesman Keir Starmer is suddenly one of the most important figures in British politics.

Labour Leader Jeremy Corbyn is increasingly absent or ignored. In his stead, MPs are turning to the U.K.'s forensic former chief prosecutor as their last hope of serious resistance to a hard Brexit. Despite only being elected in 2015, what Starmer thinks about Brexit now matters. His ability to build and cajole parliamentary opposition to the government will shape Britain’s relationship with Europe for decades to come.

In the most comprehensive explanation of his position on Brexit to date, Starmer told POLITICO that Britain's membership of the single market will have to "lapse," that Labour will push for "the fullest possible” tariff-free access to European markets, and that any new deal with Brussels will require Westminster to have some control over who comes to work in the U.K.

Setting a course at odds with his leader, Starmer argued that immigration has been too high and said Labour must support “some change to the way freedom of movement rules operate” as part of the Brexit negotiations.

Starmer is also open to the U.K. leaving the jurisdiction of the European Court of Justice — the prime minister’s core demand — as long as another body is established to settle disputes between Britain and the EU.

In all, it suggests Labour is flexible on the practicalities of Brexit, but only within certain red lines, which the party is not prepared to cross. “If she’s out, out, out then there’s no compromise,” Starmer insisted. A total, clean break from the EU, its single market and customs union is not acceptable to him. If May does not compromise, Starmer said he sees no possibility of consensus.

Starmer sounded prepared to play hardball in the upcoming skirmish over Article 50 in the House of Commons sparked by Thursday’s court ruling. He is adamant Labour has no intention of blocking Brexit in any vote, but is determined to exact its pound of flesh. “We have been pressing the government to disclose its opening negotiating strategy and we will continue to press for that," he said. "The broad objectives have to be disclosed.”

The Conservatives have a working majority of 15 in Parliament, and some of their own vocally supported Remain and prefer a softer form of Brexit. While most MPs regardless of party accept that Britain will leave the EU, May doesn't have a clear mandate for what that means. That opens the way for someone like Starmer to influence the process in the months to come by building coalitions across the aisles.

Labour's Brexit spokesman said the government cannot expect MPs to “debate Article 50 in a vacuum” and expects the prime minister to publish a document setting out Britain's opening position by January.

Starmer’s intervention points to an emerging political consensus — although far from universally accepted — on the central questions of Britain’s exit from the European Union. The battle, as he sees it, is no longer to keep Britain in the single market, to preserve free movement of people or retain the authority of the European Court of Justice over British law.

But to hear him, none of this grants May a mandate for a "hard" Brexit and, within the parameters of a British political landscape upended by June's referendum, Starmer intends to make sure that the prime minister's negotiating hand does not go unchecked.

Understanding Brussels

Starmer, the son of a working class, Labour-supporting family, came into politics late. The 54-year-old rose quickly after his election in May 2015 to become one of Corbyn's junior shadow ministers. He resigned after the EU referendum, arguing that Labour needed a new leader to guide the party through the Brexit negotiations.

He insisted that the opposition must not set its sights on the next general election but must focus on influencing discussions with Brussels that will shape the country for “decades.”

“If you look at Brexit, the future of this country is going to be determined over the next few months and years and the Labour party has to put in very effective opposition and challenge, both for the Labour party and for the country,” he said in his Westminster office overlooking the River Thames. “Simply setting out what a Labour government might do in 2020 is not an option on Brexit. It’s what the Labour Party does right now that really matters.”

"It’s what the Labour Party does right now that really matters" — Keir Starmer, Labour’s Brexit spokesman

Despite being dismissed by the Euroskeptic former cabinet minister Iain Duncan Smith as a “second-rate lawyer,” Starmer is highly rated at the top of government and commands respect from many former Remainers on the Conservative benches. He is viewed as a more formidable opponent for the prime minister than many of Corbyn's crop of inexperienced frontbenchers.

During his time as the U.K.'s director of public prosecutions between 2008 and 2013 he worked closely with May, who was home secretary from 2010. Politics aside, Starmer is fan of the prime minister — professionally at least. “She is clever, she is cautious and she knows that the language matters,” he said.

On some of the fundamental parameters of the Brexit debate, Starmer accepts that Britain's political climate has moved since June.

“Negotiation should start on the basis that there be some change to the way freedom of movement rules operate," he said. "That’s a big ask. But that ought to be the opening position.”

The idea that Britain could retain full membership of the single market, as demanded by the Open Britain campaign supported by Starmer’s Labour colleague Chuka Umunna and other leading Remain voices, is now a moot point, he said.

“There’s been a discussion about whether one should aim for membership or access. At the moment our membership is because we’re a member of the EU. That membership will have to lapse," he said. "That’s why I have used the phrase fullest possible access to the single market.”

May's mistake, he argued, is her reading of Brussels.

By setting the specifics of immigration controls up as a red line in her speech to the Conservative Party conference in October, the prime minister risked shutting down compromise before the talks even began, he said. “The mistake the government is making is, from the outset, saying it is not even worth us trying. It is not even worth us having a discussion about this,” he added.

May’s other red line in her October conference speech was taking Britain out of the jurisdiction of the ECJ. This, even more than her stance on immigration, poisoned the mood in Brussels, Starmer said.

“They interpreted that as you want out of the single market and out of the customs union. Their analysis is straightforward: It’s a rules-based system. The ECJ is the adjudicator in a rules-based system. If you don’t want the ECJ, you don’t want the rules.”

Despite his support for Remain during the referendum campaign and his conviction that May doesn't have a mandate for a "hard" Brexit, Starmer is convinced Labour must change its “state of mind” on the negotiations.

However, he believes May’s position is “shifting.”

“Her position hasn’t been consistent. It’s moved from: ‘way outside any rules,’ to ‘not binary choices,’ so you might be in a bit for some sectors, out a bit for others,” Starmer said.

As to who polices any future relationship between Britain and the rest of the continent, Starmer believes there has to be some body enforcing the rules – but it doesn’t have to be the Luxembourg court.

He points to the EFTA court that arbitrates on trade between Norway, Iceland and Liechtenstein and the EU. “Whether it’s the ECJ or some other body, it’s hard to see how you can make any agreement of an international nature without a clause in the agreement about how you resolve disputes.”

Eyes on the prize

Despite his support for Remain during the referendum campaign and his conviction that May doesn't have a mandate for a "hard" Brexit, Starmer is convinced Labour must change its “state of mind” on the negotiations.

“We have a duty to accept and respect the outcome of the referendum on the 23rd of June," he said. "We have a duty to fight for the best possible deal or outcome for the country ... The role should be to hold the government to account, to challenge what they’re doing and to challenge it robustly, but also to bear in mind that what we want to achieve for the sake of everybody is the best outcome."

Brexit will likely dominate British politics for far longer than the two-year negotiating period allowed by Article 50. “The idea that some have advanced that this can all be sorted out within two years is pretty far-fetched,” he said.

As well as the core issues of immigration and trade, new settlements will be needed on agriculture, fisheries, security, counter-terrorism, data sharing, environmental issues and “a whole host of other issues,” he added. “There will almost certainly have to be some kind of transitional arrangements.”

With Labour looking increasingly unlikely to enter government any time soon, Starmer could be forgiven for ruing his decision to hang up his barrister's wig for the hard slog of opposition.

He’s having none of it.

“I’m really glad to be in,” he insists. “To have the opportunity to hold the government to account on some of the biggest decisions for probably 50 years is an incredible privilege and I’m very glad to be in doing that.”

Speak to many Labour MPs, and so are they. No wonder they see him as a leader in waiting.