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If anyone would know what effect the so-called Brexit could have on the currency markets, it is George Soros.

Mr. Soros made his name in 1992 when Quantum, his investment fund, pocketed more than $1 billion by betting, correctly, that the Bank of England would fail to hold the British pound in the European Exchange Rate Mechanism, a fixed-currency arrangement that predated the 1999 birth of the euro.

This time around, Mr. Soros thinks not only that leaving the European Union is a bad idea, but that “too many believe that a vote to leave the E.U. will have no effect on their personal financial position.”

Writing in The Guardian, he warned that Britain’s leaving the European Union “would have at least one very clear and immediate effect that will touch every household: the value of the pound would decline precipitously.”

“It would also have an immediate and dramatic impact on financial markets, investment, prices and jobs,” he said.

Mr. Soros predicted that the currency would decline “steeply and quickly” in the event of an exit from the European Union, a fall that would be “bigger and more disruptive than the 15 percent devaluation that occurred in September 1992.”

In 1992, Mr. Soros saw that Britain’s economic conditions made it impossible for the pound to remain at a minimum rate against the German mark. That “Black Wednesday” experience soured a generation of the British on the idea of joining a single European currency and contributed to the current anti-European Union sentiment in the country.

Were a vote to leave to send the pound as low as Mr. Soros thinks it might, he noted, then “one pound would be worth about one euro — a method of ‘joining the euro’ that nobody in Britain would want.”