“I DON'T know any successful women who haven't had a powerful sponsor in their organisation to give them their first big break,” says Avivah Wittenberg-Cox, the boss of 20-first, a consultancy that helps companies put more women into senior jobs. That sentiment is echoed by many people who work in this field. But why do women need so much help?

Many men who climb the corporate ladder have sponsors, too. Indeed, they find it easier than women to persuade a senior colleague to sponsor them. But women need help more because they are generally more reluctant to promote themselves. They are also less likely to build up useful networks of contacts.

That may help to explain why women, although they now enter white-collar jobs in much the same numbers as men in many countries, still find it so hard to get anywhere near the executive suite. A new report, “Sponsor Effect: UK”, produced by the Centre for Talent Innovation (CTI), a New York think-tank, offers a detailed picture of the female talent pipeline in Britain, based on a survey of about 2,500 graduate employees, mostly of large companies. It notes that although women in Britain account for 57% of new recruits to white-collar jobs, they make up just 17% of executive directors and a mere 4% of chief executives of the FTSE's 100 biggest companies.

It is not that the women lack ambition, says the report. No less than 79% of senior women in the sample said they aspired to a top job and 91% were keen to be promoted. Nor, say the authors, are they necessarily held back by family responsibilities: nearly two in five of those aged 40 or over had no children. Three in five of the over-40s did have children, and talented women who quit work to raise kids are not included in the sample.

Still, the survey's main finding is striking. Only 16% of the sample had sponsors, defined as people several levels above them who give them career advice, introduce them to contacts and help them get promotions. Having a sponsor dramatically improves a woman's career prospects.

The British report is a follow-on from a similar study by the CTI on America, “The Sponsor Effect”, published last year by the Harvard Business Review. The CTI is now working on India; Germany may come after that. It is also preparing a “road map” to make it easier for women everywhere to find sponsors.

Sylvia Ann Hewlett, the CTI's boss, who co-wrote the report, says there are some differences between countries (for example, American male bosses are more reluctant than British ones to sponsor younger women, lest they be suspected of an affair, which could wreck their career). But the broad picture is similar everywhere: women underestimate the importance of sponsorship and fail to cultivate business relationships effectively. This may be true, but networking takes time, often after hours. Are women with children equally willing to put in that extra time?

Drawing attention to the problems of women in upper management makes a useful change from the usual lament about the lack of women on boards. The scarcity of both is a symptom of something bigger: a lot of employers neither know nor care how many senior women they have working for them. Studies show a correlation between more women in senior positions and superior company performance, though it is hard to know whether more sexual diversity improves performance or top-notch firms promote more women.

How much can sponsorship help? Ms Wittenberg-Cox is glum. She says there have been so many initiatives that many firms suffer from diversity fatigue, but none has dealt with “the appalling reality of the pipeline”. The only remedy, she reckons, is a change of thinking at the top.