London:   If Britain wakes up on Friday morning to the news that it has voted itself out of the European Union, about the only thing that everyone is sure to agree on is that the nation faces a protracted political and legal mess.

The process of decoupling would officially begin only when the British government chooses to invoke a previously unused provision of the bloc's governing treaty, known as Article 50, that sets out the basics of the withdrawal process.

British prime minister David Cameron at a "Remain" rally in Birmingham on Wednesday. Photo: Bloomberg

The most critical element of Article 50 is that, once invoked, it sets a two-year deadline for a negotiated departure. Beyond that, no one really knows how the process would work, since no country has ever left the union.

Moreover, it is up to the British government when to invoke Article 50, and it is not entirely clear whether Prime Minister David Cameron, who has led the campaign to stay, would stick to his stated plan to invoke it immediately if the country votes to leave.

In legal terms, the British government is not even bound by the result of Thursday's referendum, which is generally considered a toss-up at this point. In a report for the Constitution Society, Richard Gordon and Rowena Moffat said that "The government could, in strict law, choose to ignore it."

David Cameron campaigning for Britain to stay in the EU. Photo: Bloomberg

Most members in Parliament - including the majority of the governing Conservatives in the House of Commons and nearly all of the opposition Labour Party - are also opposed to leaving the bloc, though Parliament would probably not get a direct vote on whether or when to invoke Article 50.

Politically, however, it would be almost impossible to overlook the first plebiscite on Britain's place in Europe in 41 years. "Given the constitutional significance of the issue at stake," the report's authors say, "it is inconceivable that the government could choose not to be bound by the result."

In fact, one of the few certainties about a vote in favour of Britain leaving the EU, known as Brexit, is that initially, at least, it will plunge capitals on both sides of the English Channel, but in Britain in particular, into complex negotiations and political jockeying that could last for years.

British MP Boris Johnson, voice of the 'Leave' campaign. Photo: Bloomberg

Despite Cameron's plans to invoke Article 50 swiftly after the vote, he would face pressure to delay starting the two-year clock from those in his party who favour leaving.

Their thinking is that before starting the clock, Britain should start informally negotiating a new trade deal with the EU in tandem with the terms of Britain's departure from the bloc. They suggest that Britain would lose considerable leverage in negotiating a new trade deal once it was outside the bloc, and that it could get a better trade deal as part of a negotiation that encompasses all aspects of the new cross-Channel relationship.

Once the two-year Article 50 term expires, Britain would be outside the single European market for services and become subject to possible tariffs on goods. The pro-departure camp does not want to negotiate a new trade pact with that clock ticking.

UK Independence Party leader Nigel Farage campaigning for Britain to leave the EU. Photo: Bloomberg

But while Britain might want to move slowly to leave Europe, countries like France and Germany would want to move swiftly, to reduce Britain's leverage. They can also be expected to take no prisoners in the negotiations, in an effort to limit political contagion by making a tough example of Britain for other member states.

Yet there appears to be no mechanism to force Britain to invoke Article 50 and set the two-year clock running.

Cameron's assurances that he would do so in the event of a Brexit vote may not count for much, in that he may not survive such an outcome. Were he to quit, it would take the Conservative Party several weeks at least to select a successor. If he loses the referendum but decides to try to remain prime minister, as he has said he would, Britain could be consumed by political maneuvering for weeks or months, postponing a decision on how to proceed.

UK Labour party leader Jeremy Corbyn on Wednesday at a London rally to keep Britain in the EU. Photo: Bloomberg

"We don't know who's going to be in charge," said Anand Menon, professor of European politics and foreign affairs at King's College London. "The uncertainty extends to who's going to be leading this show."

Some Brexit supporters suggest that they could negotiate a departure without using Article 50. By contrast, in Brussels, there is discussion of somehow forcing the British to invoke it. Most legal experts say it would be impossible to avoid.

Whatever Conservative government emerges would have to decide what kind of relationship to seek with the European Union, and get the British Parliament on its side for eventual ratification of a new arrangement covering trade and immigration, among other issues.

The problem is, fewer than a third of the current Parliament members support leaving the bloc. Stephen Kinnock, an opposition Labour Party MP, has said that MPs might press for a relationship like Norway has with the EU - outside the bloc but still having access to its single market.

However, Norway not only pays into the bloc but also accepts the free movement of workers - two of the biggest and most emotional arguments Brexit supporters have made against membership in the EU.

Analysts say that the arguments in Parliament could become so polarised that the government might be forced to seek new elections. But that would require changing a recently passed law on elections, and even then a new Parliament might still be hopelessly divided.

Roger Liddle, a pro-EU member of the House of Lords and a chairman of the Policy Network research institute, said, "Even if, as is likely, within weeks of a 'leave' vote we would have a new Brexit government with a new prime minister, which may be reinforced by a general election victory within six to nine months, it is very unlikely that a majority in either House of Parliament could be found for a credible leave option."

A vote to leave the bloc would put Britain in a worse position to curb European migration until it actually departed. In February Cameron negotiated limits on welfare payments as a disincentive to some European migrants, but this concession is conditional on a vote to remain. Not only would this deal be moot in the event of a Brexit vote, but European citizens might race to enter Britain before the gates are closed.

Chris Grayling, a Cabinet minister campaigning to leave the bloc, has proposed quick legislation to end the right of free movement before Britain leaves formally, something that would put Britain in breach of EU law.

While European law would be hard to enforce on a country in the bloc's departure lounge, it is unclear whether British MPs would approve such a legally contentious step in any case.

Even if they did, that would complicate exit negotiations and could provoke retaliatory measures from Continental Europe.

If a deal can be reached within the two years, it may need to be ratified in all 28 member nations and perhaps approved by the Parliament in Scotland, where all major parties want to remain in the bloc.

Some of those not normally given to hyperbole have deep misgivings about what could go wrong while disentangling Britain from four decades of European integration.

Sir David Edward, a former judge at the European Court of Justice and professor emeritus at Edinburgh University, warned a committee of the House of Lords: "The long-term ghastliness of the legal complications is almost unimaginable". 

New York Times