Investors are piling into cautious investments in a bid to Brexit-proof their portfolios as new polls suggest the upcoming EU referendum may be too close to call.

UK companies could be facing a fall in the value of sterling, which has been fluctuating dramatically in response to polls showing a slight lead for the Leave camp. Sterling hit a three-week low against the dollar on Monday, dropping 1.5 cents at one point in the latest round of currency volatility sparked by uncertainty about the referendum outcome. It followed a poll by Opinium and the Observer newspaper which found 43 per cent of the electorate were in favour of leaving the EU, with 40 per cent wishing to remain. Another survey conducted by Yougov gave the Leave vote an even greater lead of 45 per cent to 41 per cent.

This has helped prompt an exodus from UK equities into bonds, seen by some as a safe haven in the event of Brexit. According to the Investment Management Association, fixed income was the most popular asset class in April, with net retail sales of £679 million, and the bestselling fund sector was Targeted Absolute Return, hitting sales of £742m. By contrast, UK equities saw a substantial outflow of £310m.

Adam Laird, senior analyst at execution-only stockbroker Hargreaves Lansdown, said: “The popularity of bonds and absolute return funds show us investors are still nervous. Volatility from the start of 2016 has knocked investors’ confidence and investors are fearful of the Brexit vote.

“Fixed income and absolute return funds are generally more cautious sectors which may hold up better around the referendum. Investing globally may reduce risk by spreading assets across many different regions.”

Anthony Gillham, co-investment director of multi-asset at Old Mutual Global Investors, said gilts would probably perform well in the event of Brexit, with yields dropping even lower, as investors seek to hold haven assets and price in “more sluggish” economic growth. He said: “Company debt prices might fluctuate a little in the near term, but ultimately we believe the eventual result of slow, low growth should be favourable for such bonds.”

Mr Gillam added that OM would favour corporate bonds issued by companies with “good” credit ratings if Britain votes to leave.

Larger British companies may not be as vulnerable to a Brexit shock as smaller firms, with many investors now turning towards dividend-producing companies Indeed, Mr Laird said it was "worth noting" that UK Equity Income was the third best-selling sector in April, netting sales of £342m.

An analysis of economic exposure to Brexit by Old Mutual found UK large-cap stocks had far more exposure to international markets outside the EU, amounting to 58.1 per cent (compared to 31.8 per cent for small cap) as well as being far less domestically focused (23.9 per cent versus 55.2 per cent for small cap). The report said: “If sterling weakens considerably post Brexit vs the Euro and other continental European currencies, the positive impact on the revenues of UK listed companies should be more muted the smaller the company is.”

Chris White, head of UK equities at Premier Asset Management, said highly defensive stocks such as Imperial Tobacco, Unilever and British American Tobacco would most likely get a post-Brexit bounce. “A portfolio positioned to take advantage of Brexit would be more weighted towards internationally orientated companies and consumer staples that outperform even if they’re trading on expensive valuations.” However, he urged caution over utilities like SSE and Centrica, saying they could “come under the cloud” of a second Scottish referendum (a possible consequence of Brexit).

Russ Mould, investment director at AJ Bell, said lessons could be learned from how markets reacted to Black Wednesday in 1992, when a devalued pound saw Britain crash out of the European Exchange Rate Mechanism. “Financials did well as the government was able to take interest rates lower, something that is less likely this time, but the prominence of big exporting and dollar-earning sectors like technology, aerospace & defence, electricals & electronics and industrial metals stands out.” He has suggested BAE systems, international rental company Ashtead and Wolseley, a distributor of plumbing and heating products, as stocks to watch should Britain vote to leave.

Mr Mould also suggested gold as the true safe haven for investors in the event of Brexit. “DIY investors that have already increased exposure to gold will have done very nicely as it is up 18 per cent in 2016. If Brexit were to put further pressure on economic growth and interest rates did fall further, leaving gold cheaper to hold than cash, some investors may seek a real asset alternative to central bank-controlled paper money.”

Tom Stevenson, investment director for personal investing at Fidelity, said investors should keep their eye on long-term goals rather than short-term headlines.

“Wholesale moves in and out of the market are rarely a good idea. Over-trading always eats into investment returns and is best avoided. Investors have been factoring in the possibility of an EU exit for some months now so there is a real possibility that a vote to stay in would result in a big rally in markets. Being out of the market this summer looks riskier than ever.”

He recommends balanced funds that combine defensive and growth-focused assets as well as drip feeding money into markets.“This means you end up buying more units when prices are low and less when they are high. The more short-term volatility there happens to be, the greater the effect is.”