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Markets in London are bracing for what could be a wild ride in everything from foreign-exchange to stock trading as Britin votes on European Union membership.

Voting booths will close at 10 p.m. British time Thursday (3 p.m. MDT), and the City's traders could face anything from a market maelstrom to a whimper as results trickle in during the early morning hours on Friday.

Bank of America Corp. expects a 10 percent downward jolt in equities if the country votes to leave the EU. Billionaire investor George Soros warned that the pound, the bellwether trade for Brexit watchers, could plunge more than 20 percent should voters back leaving.

ICAP operates vital Treasury- and currency-trading venues and will have extra staff on hand. Bats Global Markets Inc., which runs the biggest pan-European stock market, has tested its systems to withstand volumes that are multiples beyond what it has experienced.

Euronext and Tradeweb Markets are also taking extra measures, while banks have warned clients that some services may be limited.

"Everyone is certainly beefing up their staffing around the time of the results and the hours that follow," said Michael O'Brien, director of global trading at Boston-based mutual-fund company Eaton Vance Corp., who plans to stay on his company's trading floor until the results are known. "Banks are all doing it, I suspect most asset managers are doing it, and we're no different."

Recent market gains signal a "Leave" vote would be a surprise result for traders, even with polls suggesting the outcome is too close to call. That could test the region's systems for buying and selling financial assets.

Retail currency broker FXCM Inc. lost more than $200 million after the Swiss central bank let the franc trade freely against the euro in Jan 2015, triggering one of the biggest market upheavals in recent history.

Some Wall Street banks sought to renege on trades with clients after the event. FXCM said in a notice to its customers that it's raising margin requirements on some currencies, including the pound and the euro, this week.

Electronic-trading platform ParFX says the currency venue will have extra trading and IT staff available if needed, and has been in regular contact with customers in the run-up to the vote. Lessons have been learned since last year's Swiss franc shock, according to ParFX Chief Executive Officer Dan Marcus.

"Market participants are now better prepared for an explosion of volatility and are likely to err on the side of caution in the face of significantly higher trading activity following the EU referendum vote," he said.

Tradeweb, which facilitates trading in European government bonds, Treasuries, interest-rate swaps, among other asset classes, says it will open early, at 4 a.m. in London, to accommodate a possible surge in early morning volume.

London's stock markets are designed to automatically pause after excessive price swings. Bats says equities on its European platform are subject to price collars, which reject orders that stray too far from previous reference prices.

The thresholds range from about 5 percent to 10 percent.

Euronext, which operates markets in Paris and Amsterdam, also has circuit breakers. To cope with heavy trading this week, it has widened price limits for derivatives and may relieve market makers of some obligations.

The London Stock Exchange, which plans to open as usual on Friday, has its own thresholds ranging from about 3 percent to 10 percent or more.

If orders exceed that range, the stock automatically goes into an auction before resuming trading. The venue has accounted for about 62 percent of public U.K. trading this month, compared with 24 percent for Bats, according to Bats data.

Some key trading platforms are run by banks, which also play a vital role in making markets.

Bank of America reminded customers on Monday that extreme volatility might delay trades and that it's not obligated to accept orders, according to a client notice seen by Bloomberg News.

Societe Generale has also cautioned clients currency liquidity and pricing could be constrained.