WASHINGTON — Already the pound has fluctuated widely, money has moved into bonds, London stocks have gyrated and central banks in the U.S. and Europe are looking at contingency plans to stave off market shocks if Britain votes Thursday to leave the European Union.

An overreaction? Perhaps, but a strong consensus among policymakers, economists and investors says that the damage from Britain exiting the EU — “Brexit,” it's dubbed — would be significant. A decision to leave would probably push Britain into recession, add impetus to secession movements across Europe and jolt financial markets around the world.

Britain could lose 1 percent to 6 percent of its annual economic output, a dozen major U.S. and European banking firms and international economic groups have estimated, although supporters of the “Leave” campaign dispute that.

Despite the risk, the average of recent polls shows the two sides essentially tied, with a dwindling number of undecided voters potentially holding the key to the referendum. A poll released Sunday showing the “Remain” side pulling slightly ahead, although still well within the survey's margin of error, sent markets soaring Monday.

Many on this side of the Atlantic are on edge too. The Federal Reserve held off raising interest rates last week in part because of the risk of a Brexit.

The British referendum has added to uncertainties from the coming presidential election and lingering anxieties about China's economic slowdown, which threw markets into turmoil earlier this year.

“What I'm concerned about is the unpredictable,” said Hal Scott, a Harvard law professor and expert on international finance and securities regulations. His biggest worry about a Brexit is the possibility it could set off a panic that leads to a global financial crisis.

There's little doubt that a decision to leave would cause severe stress for the United Kingdom, the world's fifth-largest economy, and it would strain an already weary Europe, economically and politically, and ultimately lap onto the shores of America.

The British pound has slid about 3 percent against the dollar since the start of this month, half of that June 10 when a poll indicated a slim majority favoring the “Leave” campaign. Economist Mark Zandi of Moody's Analytics predicts the British currency would fall as much as 20 percent — to about $1.15 — in the immediate days after a Brexit.

A sharp drop in the pound, by itself, would tend to boost Britain's trade; weaker currency makes exports cheaper in foreign markets. The problem is that an exit from the EU would require the U.K. to negotiate fresh trade deals with partners in Europe and elsewhere, including the U.S. That would be neither quick nor easy, which means Britain could be subject to a long period of tariffs and other barriers slowing commerce.

dlee@tribune.com