The data, analysed by consultancy Ignition House for fintech EQ, found that 37% of adults with more than £10,000 in investible assets save wholly in cash accounts.
A further fifth (18%) were mostly invested in cash.
EQ (Equiniti Group), analysed data released by the FCA last week as part of its evaluation of the Retail Distribution Review and the Financial Advice Market Review.
In contrast to modest savers, those with investible assets of £100,000 to £250,000 have 67% of their assets, at least partly, in investments. For those with over £250,000 in investible assets more than 80% of assets had at least some exposure to investments.
The FCA review said: "Many consumers are holding their money in cash rather than investing it, so are missing out on the potential opportunity to make their money work better for them in the longer term. Many consumers do not seek, or receive, the sort of help with their finances that would equip them to make better investment decisions.
EQ, a provider of Sharesave schemes and other products, says the review highlighted very low returns on cash savings accounts for millions of savers who were poorly served as a result.
EQ wants better access for employees to Sharesave schemes to give them more opportunity for stock market investing.
It said a Which? Report on 1,600 cash savings accounts analysed in January 2019, using data from Moneyfacts, found only 170 accounts paid more than the Consumer Price Index.
The over-use of cash savings accounts to hold money long term meant the value of savings was being eroded, EQ warned.
More than a third of those with investible assets of £50k-£100k hold the full amount in cash with a further quarter (24%) holding most of their assets in cash. A third of those with investible assets of £100k-£250k hold savings either fully in cash (15%) or mostly in cash (18%).
Paul Matthews, CEO of EQ Boardroom, said: “The FCA figures are clear evidence that too many people are leaving substantial assets in cash accounts where they are losing value. More can be done to encourage people to take their first steps into investing to make their money work harder.”