Savers could hugely accelerate their savings growth and build up £1million in cash in less than 20 years with a simple account switch.

Account holders may prefer the safer option of keeping their funds in cash ISAs but they may be missing out on much better returns with a stocks and shares ISA.

Andrew Prosser, head of Investments at InvestEngine, said: “Stocks and shares ISAs are a brilliant way for people to grow their wealth over the longer term. Historically it’s been shown that they outperform cash ISAs over time.

“This is most starkly seen through the fact that current ISA millionaires are all stocks and shares ISA holders, with the top 25 highest value ISA accounts now holding a combined £222million.

“It means the average market value of each ISA in the top 25 is £8.9 million, making each owner a multi-millionaire.”

Mr Prosser said average returns for cash ISAs over the past decade have bene just 1.21% while those with stocks and shares ISAs have bene enjoying average returns of 9.64%.

Looking at the timescales involved to reach £1million in savings, the expert said: “If you maxed out your yearly ISA contributions, assuming an average 1.2% growth for cash and 9.64% growth for stocks and shares, it would take you 39 years to reach £1 million with a cash ISA, compared to 19 years with a stocks and shares ISA.”

A person can deposit up to £20,000 each tax year into ISAs, and this allowance can be split between different types of ISA, including cash ISAs, stocks and shares ISAs and innovative finance ISAs.

The key advantage of ISAs is all your savings are tax-free, with no tax to pay on any interest earnings or investment growth.

Savers thinking of switching may be worried by the prospect that the value of their ISA savings could go down as well as up, by investing in stocks and shares.

Mr Prosser urged people to hold the course as the markets do eventually recover from dips. He said: “There will be periods of growth and there will be periods of turbulence, something which we’ve seen recently in the market with the announcement of US tariffs.

“Investors just hope to experience the former more than the latter. It’s inevitable that markets will go through rough patches which can also be caused by things like geopolitical instability, Covid-19, and deteriorations in company earnings.”

He gave the example of the 2008 financial crisis when the S&P 500 fell by more than 50% in dollar terms, but then the market went on to recover from this in the coming years.

He explained: “Anyone who invested in it at the time and stayed the course would now have investments worth significantly more than if they’d withdrawn their funds. Dealing with these downturns and riding out the jitters is what separates a great investor from the rest.”

Mr Prosser encouraged savers to see stocks and shares ISAs as a long-term investment vehicle, as the longer you leave your funds untouched, the best chance they have to grow.

He said it’s a good idea to diversify your investments as much as possible to reduce your risks and help deliver consistent performance over time.

One way to do this is with exchange traded fund (ETF) investing via ISAs, which track the performance of an entire stock market rather than individual stocks.

InvestEngine offers this service, as well as the option to build your own custom portfolio commission-free, with zero ISA fees and zero self-invested personal pension (SIPP) fees.