In fact, one of the few certainties about a vote in favor of Britain leaving the European Union, known as Brexit, is that initially, at least, it will plunge capitals on both sides of the Channel, but in Britain in particular, into complex negotiations and political jockeying that could last for years.
Despite Mr. 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 favor leaving.
Their thinking is that before starting the clock, Britain should start informally negotiating a new trade deal with the European Union 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.
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.
Mr. 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.
“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, less than a third of the current Parliament supports leaving the bloc. Stephen Kinnock, an opposition Labour Party lawmaker, has said that lawmakers might press for a relationship like Norway has with the European Union — 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 European Union.
Analysts say that the arguments in Parliament could become so polarized 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.
“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,” said Roger Liddle, a pro-European Union member of the House of Lords and a chairman of the Policy Network research institute, “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 Mr. 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 European Union law.
While European law would be hard to enforce on a country in the bloc’s departure lounge, it is unclear whether British lawmakers 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.
“The long-term ghastliness of the legal complications is almost unimaginable,” Sir David Edward, a former judge at the European Court of Justice and professor emeritus at Edinburgh University, told a committee of the House of Lords.
Professor Menon said he worries most about the political leadership if Britain leaves: “Who’s providing it, who has the authority to do anything, and whether political contagion spreads to our European partners, which then leads to a hideous, ugly, standoff before the negotiations have even started.”