What happens now that British voters have chosen to leave the European Union?

For now, the U.K. will remain a member of the EU. Thursday's referendum is not a formal, legally binding trigger for a British exit.

Instead there are various options available to the British government from a legal point of view. Which option they choose will now be guided by politics, both domestically and more broadly across the Continent.

Note that France has made it clear it is not inclined to give the Brits an easy ride. Insiders tend to think that Berlin will follow suit. That could add a hefty dose of hostility to the negotiation.

MEP Alexander Lambsdorff speaks for many in Brussels when he says negotiations should to be wrapped up by 2019 in time for the European Parliament elections.

1. The Article 50 route

European treaties already provide for a route for a member state to exit the EU. Article 50 gives EU countries two years to negotiate a “withdrawal agreement.” Leaders of the remaining 27 EU countries would issue guidelines. The Commission would probably negotiate on their behalf.

How does this happen?

The law is not clear. The British government would presumably notify other EU members — maybe via the Commission or the European Council, maybe in writing or orally — of its intention to activate Article 50.

When to notify?

No one expects Prime Minister David Cameron to send a notification “while waiting for the removal vans to arrive,” says one EU lawyer. But EU insiders say Brussels would be wary of too long a delay, not least because it wants to lose its reputation for back-room dealing.

Chancellor of the Exchequer George Osborne has said this would happen within two weeks of the referendum. However Vote Leave's Dominic Cummings told the Economist this would be "mad and like putting a gun in your mouth and pulling the trigger."

What would an exit deal cover?

At lot. For example, Britain would want transitional arrangements for EU citizens in the U.K., and for Brits on the Continent. What happens to the work status of foreign French bankers in London, or to the health insurance of grannies on the Spanish coast? The same logic would extend to myriad other areas of law.

What about market access?

Negotiators must “take into account” an exiting country’s “future relationship” with the EU. This is where Britain needs to decide if it wants a deal like the Norwegians, the Swiss or the Turks. The default option is the existing World Trade Organization framework, which is broadly what the U.S. has. Here is an overview of the possible options.

Striking a new deal

An exit agreement need only be approved by a qualified majority of EU countries — so one country could not keep Britain in EU legal limbo by vetoing the exit. Almost everyone agrees that two years would not be enough to sort out future relations. Once the two years are up, Britain is out with an agreement or without one.

If there is a deal setting out the U.K.’s future relations with the EU, it would likely touch on issues that are not strict EU competencies. That could mean that all national capitals and parliaments might also have to sign off on the withdrawal agreement.

Buying more time

Any extension needs to be approved by all remaining 27 member states. Lambsdorff, the MEP, is probably right when he says the U.K.’s best chance of securing extra time would be to request an extension the moment it files its notification.

Can Brits change their mind at the last minute?

“Article 50 is silent about the possibility or the interdiction for a member state, after having notified a decision to withdraw, to change its mind,” notes Jean-Claude Piris, who lead the Council’s legal services 1988-2010. Eventually the EU courts would have to decide.

2. Informal talks before triggering Article 50

“There is no obligation to invoke [Article 50] immediately after a vote to leave. We are in control of the process,” wrote U.K. Justice Secretary Michael Gove in an essay published by Portland Communications, a consultancy.

There is considerable precedent for EU leaders getting together behind closed-doors to strike a deal: See the Greek crisis. Yet this process is even more woolly than the Article 50 route. Who, for example, would lead negotiations and take charge of the timetable? The lack of a fixed deadline and a legal process would likely result in the negotiations meandering. Impending disaster tends to focus EU minds: Again, see again the Greek crisis.

Generally, Brussels may be unwilling to engage in more of the backroom dealing that has attracted such criticism during the referendum campaign.

Critics could argue Article 50 has been triggered anyway

EU law does not set out a formal process for activating exit negotiations. Opponents could argue the referendum itself was sufficient notification for Article 50’s clock to start ticking.

“Simply having it in the newspapers that there has been an exit is not enough for a notification,” says Alec Burnside, a Brussels-based lawyer at Cadwalader, Wickersham & Taft. But what about the U.K. prime minister’s report to the European Council of the referendum result? Lawyers could certainly try. Disgruntled U.K. Euroskeptics could even petition European courts on the grounds the EU has “failed to act.”

3. Parliament is sovereign

Assuming the British government can get a majority in the parliament, the U.K. is free to revoke the 1972 European Communities Act that gives EU law effect in the U.K. But that would leave the U.K. in legal limbo when dealing with the EU (whose legal system would still consider it a member).

4. International law could overrule EU law

A United Nations convention on treaty law says a country can break its international treaty obligations where there has been a “fundamental change in circumstances.” A majority in favor of Brexit is just such a situation, argue Frank Vibert, a visiting fellow at the London School of Economics, and Gunnar Beck, a U.K. lawyer.

“There is simply no way that the European Court of Justice would permit [EU law] to bend to international law in this manner,” says Kenneth Armstrong, a law professor at Cambridge University.

Is any of this likely?

Both methods 3 and 4 risk damaging relations with Brussels, where there is a strong preference for following the rules and avoiding legal headaches down the road.

Even an amicable deal risks major legal hurdles. Any exit deal struck outside Article 50 would risk legal challenge before the EU courts. What is more, any treaty changes would trigger a referendum in other EU countries, which could either stop the process dead and/or infuriate those on the other side of the table.

What happens to legislation in the intervening period?

The Leave campaign has proposed immediate legislation limiting the powers of the EU courts and free movement into the U.K.

But EU law still applies in the U.K pending an exit. “A bill that limits the powers of the ECJ is a plain contradiction of EU treaty obligations,” says Burnside, the EU law lawyer.

The U.K. would face court action. The Commission can and does sue governments that breach EU law — and in particular those that expressly flout it. Although the process is usually lengthy, EU lawyers insist it can act speedily where necessary. EU courts could impose daily fines under an accelerated procedure. In parallel, EU citizens could sue the U.K. government for compensation if they suffer damages as a result of conduct contrary to EU law.

However, U.K. action is unlikely to contravene a fundamental value. Member states have the power to punish other member countries that flout the EU’s fundamental values. “My understanding is that this would not be applied for restricting free movement,” says an EU lawyer.

The U.K. courts would face a constitutional crisis. Judges would have, on the one hand, the 1972 Act telling them to apply EU law and, on the other, legislation restricting it. A fundamental tenet of EU treaty law is that it trumps all national law.

“Issues of legality would be fought out in the courts, leaving judges in a very difficult situation,” Burnside says. There would be a “constitutional conflict [that] would antagonize and politicize the judiciary,” warns Armstrong, the law professor at Cambridge. Governments of Scotland and Wales could also raise constitutional concerns.

Relations with Brussels would be badly damaged. “It is hard to imagine that unilateral action to stop applying EU law, while still under a treaty obligation to so, will create a lot of goodwill in subsequent negotiations,” says Aart Loubert, a lecture in EU law at the University of Amsterdam.