Sticking to the investment theory this year would see investors out of pocket, and over the last 30 years, investors would have lost out two thirds (63%) of the time.

Analysis of the FTSE All Share between 1 May and 1 September by Fidelity International revealed that investors would have lost out on 1.06% growth in 2018, while over the past three decades, the index has given positive returns during this period 19 times.

This means anyone who invested £10,000 30 years ago in the FTSE All Share and followed this adage every summer by putting their funds into a cash account would be left with £104,149. This compares to £108,966 if they had remained invested throughout – nearly £5,000 more.

An investor who withdrew from the market and put their cash under the mattress each summer, would have turned their £10,000 into just £78,833.

‘Opportunity cost of being out of the market is greater than ever’

Tom Stevenson, investment director for personal investing at Fidelity International, said: “With the famous St. Leger Day race on the horizon, those following the old adage must surely realise that the odds of it paying off are now stacked firmly against them. The saying has been doing the rounds for decades but anyone heeding it over the past 30 years would have been worse off two thirds of the time.

“The comparison is also skewed by the fact that for much of the 30-year period it was possible to earn a reasonable income from cash on deposit. We have assumed that an investor who sold out of the market will have earned the prevailing rate of interest during the summer months. These days with deposit accounts paying next to nothing, the opportunity cost of being out of the market is greater than ever.”

Stevenson added that the markets have been beset with uncertainty and volatility this summer, given the looming threat of a US-China trade war, a lack of clarity over Brexit and economic woes in Turkey.

“Yet still, the FTSE All share has gone on to deliver a positive return between May and September.

“This tells us that trying to predict the best time to be in and out of the market is a fool’s errand, while our analysis proves that getting it wrong can severely dent your long-term investment returns,” he said.