Reuters reports that a vote to leave the EU on June 23 would spook investors by undermining post-World War Two attempts at European integration and placing a question mark over the future of the UK and its $2.9 trillion economy.

Citi, Deutsche Bank, JPMorgan, Goldman Sachs, HSBC, Barclays, Royal Bank of Scotland and Lloyds are among those banks planning to have senior staff and traders working or on call in London as results start to dribble in after polls close at 2100 GMT, according to Reuters.

It is feared that a vote to leave could unleash turmoil on foreign exchange, equity and bond markets, spoiling bets across asset classes and potentially testing the infrastructure of Western markets such as computer systems, stock exchanges and clearing houses.

Federal Reserve chair Janet Yellen has cautioned that a Brexit vote could shake financial markets and potentially push back the timing of the next rise in US interest rates.

Bank of England governor Mark Carney has said sterling could depreciate, "perhaps sharply" and some major banks have forecast an unprecedented fall to parity with the euro and as low as $1.20 in the days following any vote to leave the bloc.

The Bank of England will be staffed overnight, with senior policymakers on call if markets go into meltdown. The finance ministry would not comment on its staffing plans.

The official Vote Leave campaign argues there is no evidence that leaving the EU would weaken sterling long term, while Nigel Farage, leader of the UK Independence Party has said that even if the currency did fall, it would simply boost British exports.

Sterling - the world's fourth most traded currency - has moved sharply in recent weeks, often on the back of opinion polls.

Depending on the results from across the UK, the night of June 23 and early morning of June 24 could rank as one of the most volatile nights in the history of the London market.

"We've all seen US elections, UK general elections, we've had the Scottish referendum, the collapse of Lehman and QE (Quantitative Easing) but this is by far and away the biggest risk event that has presented itself to the UK," said Chris Huddleston, head of money markets at specialist bank Investec.

London accounts for 41% of global turnover in the $5.3 trillion-a-day foreign exchange market, more than double the turnover in the US and far more than the 3% of its closest EU competitors, France and Switzerland.

Manufacturers largely divided

Meanwhile, a survey of 900 UK manufacturing companies has revealed the industry is largely divided in its opinion about whether the UK should withdraw from the EU.

The research was carried out by UK-based specialist recruiter Manufacturing Futures, part of the recruitment company Futures.co.uk.  Business leaders from a wide range of UK manufacturing companies were asked whether they think manufacturers are better served by the UK remaining in the EU.

Of the 900 manufacturers quizzed, 55% said they think the UK should go it alone compared to 45% who think the UK is better off staying in the EU.  However 14% of respondents admitted that they could still be persuaded to change their mind about how to vote in the upcoming referendum on 23rd June.

Chris Griffin of Manufacturing Futures (pictured above) said: “Manufacturing is a key sector in the referendum debate because so many UK companies trade in the EU and rely on being able to export freely.  The UK still ranks as the 11th largest manufacturing nation in the world and, although many businesses have concerns, we are surprised the results of this poll are so close with the referendum fast approaching.

“The number of people who could still change their minds could determine the outcome of the referendum.  The remaining campaigning, from both sides, will be crucial in deciding the UK’s future.”

The poll surveyed business leaders running manufacturing companies of all shapes and sizes, across all sectors of the manufacturing industry.

Futures.co.uk employs over 40 people across offices in Leeds, Manchester, Birmingham and London.  Manufacturing Futures is the largest brand within the business, with a team of 12 and more than 20 years of experience predominantly recruiting directors, managers and engineers across all areas of the manufacturing industry.