FlagTraders and brokers across London are bracing themselves for an all-night vigil as the counting of votes in the historic referendum on whether or not Britain should remain a member of the European Union (EU), begins on Thursday night.

Senior staff and traders at banks including Citigroup, JPMorgan, Goldman Sachs and Morgan Stanley have been asked to stay overnight in the office while others are working in shifts between the British market close on Thursday and its reopening on Friday morning, the Financial Times reported.

Britons on Thursday morning went to the polls to cast their ballots in the referendum. An estimated 46,499,537 people are entitled to take part in the vote — a record number for a British poll.

The polls opened at 7 a.m. and will end at 10 p.m.

JPMorgan has booked hotel rooms for its clients in Canary Wharf while Citigroup has organised taxis to make sure key sales and trading employees arrive at the offices before 4 a.m. on Friday morning.

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Online brokers and trading platforms that service retail clients are expecting a flurry of trading activity and firms such as IG Group and CMC Markets are laying on extra staff on Thursday night in anticipation.

“We have got an extra 30-40 people staggered across the night in London,” said Grant Foley, Chief Financial Officer at CMC Markets. “We have paid for people to have hotel rooms near the offices and we will be ordering in food to keep them sustained.”

Several hedge fund managers said they planned to leave the office as usual on Thursday evening but expected an early start on Friday.

“We anticipate it being a volatile day on Friday,” said David Harding, founder of quantitative asset manager Winton Capital Management and joint treasurer of the ‘Britain Stronger in Europe’ campaign.

“One way or another, the markets will move quite a lot, which will translate into quite a nervy day.”

Goldman Sachs’ private bank has told clients that the desk will be staffed all of Thursday night if they wish to trade.

Both Goldman and JPMorgan are among banks warning clients that parts of the market may become illiquid and it could be difficult to execute some trades on Friday.

While the overall picture could be clear by 3.30 a.m., the official result is not expected until around breakfast time on Friday.

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