The number almost tripled from 1,480 the previous year.

The number of investors with seven-figure pots has increased almost tenfold since 2016, according to a freedom of information request submitted to HMRC by The Openwork Partnership.

The previous record high number of ISA millionaires was 2,000, recorded by the taxman in 2019.

The top 50 ISA investors are sitting on average pots of around £8.5m, with the average ISA millionaire pot being £1,397,000.

Nearly all ISA millionaires were stocks and shares investors.

Liberal Democrat peer Lord Lee of Trafford became the first publicly declared ISA millionaire in 2003.

Setul Metha, head of partnership services at The Openwork Partnership, said: “The ISA has not only created thousands of millionaires, but it has also empowered millions of ordinary investors to build a nest-egg alongside their pension. It is perhaps the best tax-free investment wrapper in the western world.

“Of course, becoming an ISA millionaire takes time, money and a little bit of knowledge. But the rise in personal finance content online means the roadmap to millionaires’ row has never been more clearly signposted.”

The Openwork Network analysed the latest ISA figures from HMRC, data for April 2021.