Latest figures from HM Revenue & Customs (HMRC) show 80% of the 12.7 million adult ISA accounts opened in the 2015/16 tax year were cash ISAs, which was “broadly static” with the previous year.

Around £80bn was subscribed to adult ISAs in 2015/16, with just over 20% going into stocks and shares ISAs.

Investment firm Royal London said the figures “reinforce the concern that investors are holding excessive amounts in accounts with low interest rates which deliver negative real returns once account is taken of inflation”.

Currently no cash ISA product pays a rate that beats or matches inflation.

The best savers can get is 1.75% on a five-year fixed-rate ISA from Paragon Bank.

Royal London said investors have missed out on £100bn in returns over the last decade by staying in low-return cash ISAs rather than investing in a diversified stocks and shares ISA.

The HMRC figures show lower income and younger investors are more likely to favour cash.

In 2014/15, all income groups earning less than £30,000 a year were more likely to hold their ISA savings in cash-only products, whereas the reverse was true for those earning more than £30,000.

And of the £971m put into ISAs by the under 25s, £927m went into cash-only ISAs.

Helen Morrissey, personal finance specialist at Royal London, said: “Saving in cash clearly has a part to play for short-term emergencies and rainy day savings. But a combination of low interest rates and rising inflation means that money in a cash ISA is losing spending power, year after year.”

If you’re considering opening a stocks and shares ISA for the first time, there’s plenty of useful reading material in our investment section.

A good place to start is: A practical guide to moving from a cash to stocks and shares ISA