The median age of the firm’s 3,794 SIPP millionaires was 63 as of the end of March.

The total number of SIPP millionaires has grown by 20% over the past two years.

SIPP millionaires tended to hold more investments in the UK, US and gilts than non-SIPP millionaires, due to taking a more diversified view of their investments, according to the research.

Helen Morrissey, head of retirement analysis at Hargreaves Lansdown, said the research shows that more needs to be done to provide affordable childcare if we are to close the pensions gender gap.

She said: “The real fly in the ointment of this data is the lack of female SIPP millionaires. Only 10% are female. This yawning gender pension gap is down to many factors – most notably lower average pay, part-time work and time spent out of the workforce looking after family members. Over time, these factors conspire to give women smaller pension pots than men.

“Again, auto-enrolment means more women will be able to build up a pension over their working lives, but more needs to be done - such as the provision of good quality affordable childcare if we are really going to move the dial on this.”

The top funds invested in by Hargreaves Lansdown’s SIPP millionaires (in terms of assets) were Fundsmith Equity and Lindsell Train Global Equity.

The top shares held were Apple and NVIDIA Corp.