Gender diversity among Britain's biggest listed companies declined over the past year, despite evidence showing that a higher proportion of female executives could help nearly double profits.
The annual Women Count report by The Pipeline, which delivers female leadership programmes, said the number of FTSE 350 companies with no women on their executive committee increased since 2016, with eight more firms operating with all-male boards.
It also found that the proportion of female executives across those firms stayed the same at only 16%, while 65 of 791 executive directors were women - totalling less than 10%.
It means the majority of UK-listed firms are failing to reap the financial benefits of greater gender diversity.
According to The Pipeline, companies with executive committees that are at least 25% female see net profit margins nearly double compared with those with no women in leadership positions.
If all firms in the FTSE 350 performed similarly to those with a higher proportion of of female execs, those companies could have made an £5 billion all together.
Pipeline co-founder Lorna Fitzsimons said the report should be a "wake-up call" for corporate UK.
"This report is ringing a very loud alarm bell for business. With agendas dominated by Brexit, the focus on gender diversity at senior levels has been slipping.
"Women Count shows there is an increase in profit for companies who have over 25% women on their executive committees. In this post Brexit era, can this return be ignored by other companies?"
The retail sector was singled out as having been "very successful" at appointing women to executive roles, with one in two companies boasting committees that are at least 25% female.
Meanwhile, Resource and utility firms - particularly electricity, oil, gas, steam, waste and water companies - were within the top quartile of firms with 25% of their exec team made up by women, the proportion of women in profit and loss roles and executives on their main boards.
"Nevertheless, this only really demonstrates that this sector performs better than others," a summary of the report explained.
"The fact remains that the number of women in senior roles across the FTSE 350 is low and every sector can do more to improve."
The report also found that overall, most female executives are appointed to "functional" roles like human resources, marketing, legal and compliance.
Meanwhile, the percentage of women in profit and loss roles amongst FTSE 350 executive committees has dropped from 38% to 35%.
Ms Fitzsimons said firms will have to do more to address gender diversity if they hope to meet Government targets that aim to see women make up at least 33% of directorships by 2020.
"This is not an equality issue. This is an economic issue and there are signs we are starting to slip back.
"This report should serve as a call to arms for us all."