THE FTSE 100 Index rose by 453 points – the equivalent of 6.42 per cent - in April, ending the month at its highest level since the end of January.

Some investors might therefore be wondering whether now is the time to cash in their portfolios and follow the old stock market adage to “sell in May and go away, don’t come back ‘til St Leger Day”.

The saying is believed to date from a time when stockbrokers left the City for the summer to enjoy ‘The Season’, a period of sporting and social events including Royal Ascot, Wimbledon, the Henley Royal Regatta and Cowes Week, ending with the St Leger flat race in Doncaster in mid-September.

But the market today is very different from in the past, with Justin Urquhart Stewart, co-founder and head of corporate development at investment manager 7IM noting that “there is no evidence to suggest summer is a particularly bad time for markets”.

“The FTSE All-Share has been in positive territory about 60% of the time in the summer months over the last 30 years,” he said.

Share prices as measured by the capital return on the FTSE All-Share Index have declined and risen an equal number of times - 16 each - during the period between May to mid-September over the last 32 years, according to Bestinvest, the online investment platform for fund and share dealing.

However, once dividend payments are factored in, UK stock market returns have been positive 66% of the time during the summer, compared to 78% of the time across full calendar years.

There have, however, been significant variations in returns during the summer. Firm believers in selling in May can point to seven brutal summer sell-offs since 1986 when the capital returns on UK shares posted double-digit declines: 1992 (-13.4%); 1998 (-13.4%); 2001 (-19.2%); 2002 (-22.1%); 2008 (-13.0%); 2011 (-12.3%); and 2015 (-10.3%).

However, they should not ignore the six soaring summers of 1987 (+12.2%), 1989 (+10.1%), 1995 (+11.1%), 2003 (+12.7%), 2005 (+12.7%) and 2009 (+19.3%).

Jason Hollands, managing director of Bestinvest, said: “While market returns in the summer months can be unpredictable, systematically exiting the market during this period does not convincingly stack up as a strategy in the modern age. Pursuing such an approach could also clock up transaction costs and capital gains tax liabilities.”

Also, companies typically pay their final dividends in the summer months so anyone thinking of leaving the market for the summer should take care not to lose their dividend entitlement.

If you had invested £10,000 in the FTSE All-Share 30 years ago and remained invested throughout the whole time you would now have a pot worth £128,033.

If, on the other hand, you had followed the adage, sold in May and bought back in September every year, you would have a portfolio worth £126,950, according to investment business Fidelity International.

Tom Stevenson, investment director for personal investing at Fidelity, said: “Our analysis shows this old market maxim really is a bit of a non-starter and the smart money would have been on keeping your money invested in the market throughout the past three decades. Just missing a handful of the best days in the market can seriously compromise your long-term returns.”

Online trading platform Interactive Investor has analysed the performance of both the FTSE 100 and FTSE All-Share over the period April 30 and September 15 for the years between 1986 and 2017.

Moira O’Neill, head of personal finance at Interactive Investor, said: “Interactive investor’s research shows that 14 of the 32 time periods in question, May to mid-September, have been negative for both the FTSE 100 and FTSE All-Share indices,” she said. “The market is a very different place compared to the days of old – now, the City never sleeps.”

Despite this, Ms O’Neill said that investors should regularly review their portfolios.

“Every investor should have a spring clean once or twice a year with a view to rebalancing their portfolio by selling some of the funds and stocks that have done well and buying some that haven’t,” she said. “It’s an important way of keeping your level of risk the same and ensuring there is diversification across your portfolio, so you are not over-exposed to one stock.”