UK Report Says Leaving EU Would Cause Decade of Uncertainty

February 29, 2016 Updated: February 29, 2016

LONDON—A new report by British civil servants concludes that leaving the European Union would touch off a decade of uncertainty because of the complex process of agreeing to terms to leave.

The report released Monday said that millions would be affected as the U.K. takes a decade to extricate itself from the 28-nation bloc. First there would be the process of negotiating the exit, then future arrangements with the EU and ultimately trade deals outside of the bloc.

“In short, a vote to leave the EU would be the start, not the end, of a process,” the report said in bold letters in an executive summary. “It could lead to up to a decade or more of uncertainty.”

The report is important in that it reflects the assessment of Britain’s apolitical and respected civil service. The view is intended to be impartial, though the report’s conclusions generally mirror the concerns of Prime Minister David Cameron that an exit would put the country into unknown territory.

Exiting the EU involves taking the unprecedented step of invoking Article 50 of the Treaty on European Union. Though Greenland left an earlier, more limited version of the bloc in 1985, invoking Article 50 has not happened before.

Much debate has centered on what would actually happen should Britain vote to leave on June 23.

Backers of the exit campaign have repeatedly argued that their opponents are scaremongering. London Mayor Boris Johnson, for example, used his column in the Daily Telegraph to warn of “questionable assertions” about the impact of a British EU exit — or Brexit — on the economy and national security.

“In every case the message is that Brexit is simply too scary; and the reality is that these threats are so wildly exaggerated as to be nonsense,” Johnson wrote. “Indeed I am ever more convinced that the real risk is to sit back and do nothing, to remain inertly and complacently in an unreformed EU that is hell-bent on a federal project over which we have no control.”

He chastised Britain’s Treasury — under the guidance of political rival George Osborne — for persuading others in the Group of 20 of the world’s biggest economies to include a warning in a post-summit communique on Saturday that having Britain leave the EU “is among the biggest economic dangers this year.”

“Surely the first time any country has used an international forum actively to talk up threats to its own economic prospects,” Johnson wrote.