LONDON — Britain is one of Europe's strongest economic performers, having emerged from the financial crisis of the last decade faster and less burdened by debt than competitors like France, Italy and Spain. Unemployment is at a 10-year low of 5.1 percent. London, vibrant and second only to New York as a global financial hub, hums a siren song to the young and ambitious across Europe.
Those campaigning for Britain to leave the European Union in a national referendum on June 23 focus their arguments on reclaiming national sovereignty and reasserting national identity in the face of immigration. But proponents of exit are also now confronting the challenge of convincing the country that any freedom it gains from divorcing the Continent would not be offset by the risks to Britain's prosperity.
Estimates of the economic impact of a "Brexit" vary, because no one knows what terms would be negotiated in what would most likely be a yearslong divorce. But that very inability to foresee the consequences has become a factor in the debate, especially since the value of the pound started to soften over concern about Britain's future.
Mark Carney, governor of the Bank of England, framed the stakes recently, saying the prospect of British exit is "the biggest domestic risk to financial stability because, in part, of the issues around uncertainty."
Carney was immediately attacked by "leave" proponents for playing politics and defending the position of Prime Minister David Cameron, who is leading the campaign for Britain to stay. Jacob Rees-Mogg, a Conservative member of Parliament who favors exit, responded archly by saying Carney's position was "speculative and beneath the dignity of the Bank of England."
The bitter exchange is emblematic of the passionate debate around a Brexit.
Those who favor breaking away argue that a country with close ties to the United States and the Commonwealth as well as Europe would emerge better off, freed from onerous regulations and Britain's net annual contribution of 8.5 billion pounds ($12.1 billion) to the European Union budget.
If Britons vote to leave, Article 50 of the Lisbon Treaty will come into force, giving the remaining 27 nations of the bloc up to two years to unwind their 43-year marriage to Britain and negotiate a new arrangement.
Boris Johnson, London's mayor and a prominent advocate of Brexit, insists that leaving the European Union would be "win-win for all of us," that the bloc is an anachronism that "costs us a huge amount of money and subverts our democracy," and that voters are like prisoners who are afraid to step outside "into the sunlit uplands."
But very few economists think that way; most believe that Brexit would create havoc with the pound, cut growth, damage the financial center of the City of London and provoke a lengthy period of uncertainty, with no guarantee that Britain could quickly negotiate free-trade agreements.
Within 10 or 15 years, some of these economists think, Britain would recover from a decision to leave. But the loss of economic growth in that transition period would be very hard to recapture. And the budget gain, quite small compared with the loss of gross domestic product, would be even smaller because some of the money that goes to Brussels would have to be paid directly to Britons anyway, replacing European programs with domestic versions of farm subsidies and research grants.
One of the few pro-Brexit economists, Andrew Lilico, executive director of the consulting firm Europe Economics, says the cost of leaving would be real, but would "roughly net out by around 2030."
"The costs would rise before the benefits kick in," he acknowledged, writing for Politico.eu. "But few things worth having in life come free."
Others see harsher consequences. A deal that preserved free trade with the European Union would almost certainly require — as the bloc does with Norway and Switzerland, part of what is known as the European Free Trade Association — acceptance of freedom of movement and labor and a sizable, if smaller, contribution to the European Union budget in any case.
In the case of Brexit, many economists think that the Norway model would be the least bad for Britain's economy. But according to Prime Minister Erna Solberg of Norway, Britain would be worse off, being subject to EU rules without participating in their formation.
Pier Carlo Padoan, Italy's minister of economy and finances, said the referendum was risky given the unhappy mood all over the Continent in the wake of the last economic downturn and its lingering effects.
"The Brexit story is the sophisticated U.K. version of the general dissatisfaction in Europe," he said. "The EU is not delivering growth and jobs 10 years later. People don't like the machine."
Still, he said, "Brexit is bad economically for both Britain and for the European Union," because "there's no vacuum, no withdrawal from the international system without pain." But worse would be the political impact, he said, because it could "also show that one can 'undo' membership, which is dangerous."
There is a lot of scaremongering, with some supporters of remaining in the bloc talking of the loss of 5 million jobs if Britain votes to leave. (Cameron talks of 3 million jobs dependent on EU membership, plus higher costs for ordinary goods and mortgages.)
The stakes are especially high for Britain's prominent service sector, which makes up more than 75 percent of economic output, and in particular financial services.
Multinational banks like HSBC and Goldman Sachs have warned that Brexit would mean the export of jobs and headquarters to other European countries, because London-based companies would no longer have "passporting rights" for their services in the European Union.
The Center for Economic Performance, at the London School of Economics, which sees itself neutral, says the worst-case scenario in the event of a vote to leave the bloc is a 6.3 percent to 9.5 percent reduction in GDP, "a loss of a similar size to that resulting from the global financial crisis of 2008/09." The best case, it suggests, is a loss of 2.2 percent of GDP, with the European Union imposing costs on Britain for leaving, and to discourage others from doing so.
That is a case also made by Leszek Balcerowicz, a former Polish deputy prime minister. To deter populists across Europe who might want to push their countries to follow Britain out the door, he said, the European Union, even against its own economic interests, would punish Britain.