LONDON (AP) — Advocates of leaving the European Union have not thought through its economic impact, British Prime Minister David Cameron said Wednesday, warning that an exit would spark "a decade of uncertainty."

Cameron said it could take years for Britain to negotiate new trade deals with the other 27 EU members. Collectively, they account for more than 40 percent of U.K. trade.

Britain will vote June 23 on whether to remain in the bloc it joined more than 40 years ago. Leaders of the "leave" campaign say the U.K. would soon forge new trade agreements, and some point to a Canada-EU trade deal as a model.

Speaking to a committee of lawmakers at a regular question-and-answer session, Cameron said the Canada deal was seven years in the making and is still not in place. It also does not apply to services, which account for a large chunk of Britain's business with the EU.

He said it was "a good deal for Canada, thousands of miles away from the continent of Europe. It's not a good deal for Britain."

Cameron and other leaders of the "remain" campaign have repeatedly warned that a British EU exit, or Brexit, would hurt the economy. They're backed by major economic organizations including the International Monetary Fund, and by President Barack Obama, who said last month that Britain outside the EU would go to the "back of the queue" for a U.S. trade deal.

Some economists disagree — saying the British economy could benefit if freed from EU red tape — and "leave" campaigners accuse their opponents of scaremongering.

But Cameron said he didn't want voters to wake up on June 24 and say they hadn't been warned.

"We would be potentially looking at a decade of uncertainty," Cameron said.