"The last time ISA sales were this high the UK economy was at its peak, before the tech bubble – these figures could be a warning," Tim Cockerill, head of research at financial adviser Rowan said.
The warning came amidst a flux of market volatility, as poor macro data from the US and China dampened investor sentiment. Managers and IFAs have been warning of a return to recession in recent weeks.
Performance of FTSE All Share vs CBOE SPX Volatility VIX over 3-mths
Source: Financial Express Analytics
Cockerill also said the ISA sales, which totalled £503m for the month, signified a shift in investment strategy since the early noughties.
"In the tech boom there were a lot of people investing for the first time, swept away by how well the tech companies had done. There has been a lot of experience gained between then and now, and these high ISA sales figures indicate a more cautious investor," he noted.
The IMA figures back up this premise; while net ISA sales in May were the highest since 2000, total net fund sales for the month were the lowest since November 2008, falling below £1bn for the first time in 18 months.
"This isn't surprising; investors have become more cautious, while talk of tax increases has put a renewed focus on investing in tax efficient wrappers such as ISAs," Patrick Connolly, head of communications at AWD Chase de Vere said.
Other intermediaries agree with this shift in sentiment, particularly since the tax changes announced in the emergency budget last month.
Alan Easter, director of discount broker Willis Owen, said in a note: "Investors are right to take advantage of the tax free benefits of investing within an ISA. With taxes on the rise it is essential to take every tax break you can. Although the CGT rise was not as bad as first feared, using the annual ISA allowance should be a priority for all investors."
The figures came as the Office of Fair Trading (OFT) slammed banks for dragging their heels when customers switch ISA accounts, leaving investors short changed. From 31 December, banks will have to transfer cash ISAs within 15 days, down from the current 23 days.