FTSE 350 companies will need to give around 40% of all new leadership roles to women over the next two years if they are to meet targets for female representation on boards by 2020.
That is according to a government-backed review, which has challenged the UK's largest firms to have a third of all board positions held by women by the end of the decade.
The review is today at its halfway point after being launched in 2016, but has found that female representation on boards still lags at just 25.5% for FTSE 350 firms, with 10 companies only having men in top positions.
This comes after research found that increasing gender equality could add £150bn to UK GDP in 2025 through enhanced productivity and business reputation.
"For FTSE 350 to fall short of this target is just not good enough," said CMI Women chair, Heather Melville OBE. "Why is it that in 2018 achieving gender diversity in leadership teams is not a business priority?
"Employers lagging behind risk being outperformed by 21% compared to their competitors with diverse leadership teams. FTSE 350 companies should treat this as any other business initiative."
Despite the UK's 350 biggest firms lagging behind, the review found that women now hold 29% of all FTSE 100 board and senior leadership positions - up from just 12.5% back in 2011.
Today's statistics show that if progress matches the same gains made over the last three years, then FTSE 100 companies are on track to meet the 2020 goal.
Firms failing to make enough progress are now being urged to emulate the success of more than 80 companies that have already achieved or beaten the target, including Whitbread, Diageo, and Next.
"While it is great to see there are more women at the top of Britain's largest businesses than ever before, it is clear there is still a long way to go," business secretary Andrew Griffiths, said
"The review is highlighting the benefits of everyone having an equal opportunity to reach the top, and I urge businesses to keep stepping up and championing diversity."