Savings in stocks and shares ISAs were worth £178bn in 2009/10, beating £172bn in cash ISAs, according to ONS figures.
With the prospect of market growth, the number of stocks and shares ISA accounts rose from 2,960 to 3,017 in the 12 months to 5 April.
However, average interest rates of just 0.69% meant the number of cash ISA accounts dropped from 12,234 to 11,906.
Patrick Connolly, head of communications at AWD Chase De Vere, believes it is not a surprise to see investors moving away from cash ISAs.
However, he stresses they are not necessarily going for equities, but instead other assets such as property and bonds.
He adds: "There is the risk that people, moving away from cash, take too high a risk, although it is not something we have seen.
"I think people will switch back to cash when interest rates improve, although we are looking at the end of next year for that."
Lack of innovation for solutions
Some 2,000 consumers affected
Achievements, charity work and other happy snippets
Appetite has suffered since Brexit vote
'Failure to pay attention can result in enforcement'