An old boys' network that only likes to promote those who "look, think and speak like the current leadership" is preventing thousands of women from accessing top jobs, the leader of an influential group of girls' schools will say tomorrow.
Helen Fraser, chief executive of the Girls' Day School Trust, will tell her annual conference that quotas must be introduced for women on boards of top companies to avoid women between the ages of 30 and 50 facing "lost decades" when men overtake them in the workplace.
In every sector of society from politics to the police, there are fewer women than men in top leadership positions - despite the fact girls now outperform boys at every level from national curriculum tests at 11 to the awarding of university degrees, she will say.
""You sometimes hear the excuse 'but they all go off and have babies'," Ms Fraser will add. "That's not the real argument. It's just that - an excuse - we need to look deeper.
"We need to look at what society is doing that is holding women back - from a long hours culture in too many workplaces to an old boys' network that only likes to promote individuals who look, speak and think like the current leadership."
In her address, she will also argue that - in their first years of employment - girls often do better because of their superior qualifications, but will add: "And then what happens - why do there seem to be for so many women two 'lost decades' between 30 and 50 when they are overtaken by men?
"Are the very qualities that get them so brilliantly through education and the first stages of work (attentive, eager to learn, persistent, independent, flexible and organised) somehow disadvantaging them in the battle to climb a corporate or political ladder?
"With the possible exception of boardroom quotas, I believe we've probably gone as far as necessary in terms of legislating for equality ... As a pragmatist I am very much in favour of 'what works' and it is clear to me that quotas for women would make a big difference in a short space of time.
"I don't think this would need to be a permanent measure but a short-term one until the representation of women reaches a tipping point."
The Girls' Day School Trust represents 24 independent schools and two state-financed academies teaching around 20,000 pupils between the ages of three and 18.