A study from market research firm Kantar TNS discovered about £14.3bn in investments was held by women, while £29.3bn was held by men.
The study, conducted in Q3 this year, had looked at the holdings of millennial and generation X adults (aged 21 to 53).
It had surveyed a nationally representative sample of 1,220 men and 1,276 women. Of these 724 were millennials and 659 were generation X.
The study found just 26 per cent of millennial women thought of themselves as having a high level of financial engagement, compared with 55 per cent of men.
About 4.4 per cent of millennial women had an investment product compared with 7.3 per cent of millennial men and 9.6 per cent of generation X women owned an investment product compared with 16.1 per cent of men.
In value terms, £3.6bn was invested by millennial women, those born between 1981 and 1997, while £7.9bn was invested by men of the same age group.
The picture was similar for generation X women, born between 1965 and 1980. They were found to have investments valued at £10.7bn, while men of the same age had £21.4bn invested.
Kantar said the findings suggested investment providers were failing to connect with a female audience.
Alice Moss, managing director for qualitative research at Kantar TNS, said: "We found that there is a fundamental difference between women and men’s levels of self-esteem and financial autonomy.
"Women can feel embarrassed about their perceived lack of knowledge of the subject and talk about it less frequently with their peers; they also have less connection with the language and marketing used by financial organisations, and have too many other priorities which push investing off their list.
"They’re also not helped by the fact that the sector has been slower than other industries in connecting with a female audience, boosting their self-esteem and reassuring them that they will not be judged."
The study had found financial independence was contributing more to men’s self-esteem than women’s.
But Ms Moss pointed out women often controlled household budgets and spending, which meant the financial sector was failing to connect with a large portion of potential investors.
She said: "The traditional approach that investment brands are taking obviously isn’t working for women. There have been some moves in the right direction but we want to see these organisations truly stepping out of their comfort zone and attempting to reach women in a new way.
"Done correctly, there’s an opportunity for something like a 'this girl can'-type campaign in the sector – and the first to take the plunge could see themselves boost not only women’s confidence, but their own bottom line."
Anna Sofat, managing director of Addidi Wealth, said: "The only targeting of women the industry does is just to present product in a pink or fluffy way, rather than address the real concerns women may have.
"Women are not so much risk averse as requiring more persuasion. They are far more likely to invest if they feel comfortable that their decision is made on a sound basis rather than just the hook of a potential good return, which tends to appeal to men.
"There is a huge gap in the market that investment companies could help fill by a more positive approach to potential female investors."