An increasing number of top female executives are opting out, leaving employers with a huge gap in their workforce and struggling to understand the resignations, according to consultants Croner.

Despite incentives like flexible working and maternity leave, intended to enable women to manage family life and career more easily, successful women abandoning their careers are citing lack of fulfilment and development opportunities.

Fears of a 'hidden brain drain' are growing as research shows financial and other tangible benefits are of no consequence to such senior, high profile women executives.

A survey sponsored by Goldman Sachs, Ernst & Young and Lehman Brothers found that four in ten female senior executives are taking a complete career break, not because of family commitments, but because of a lack of job satisfaction.

Few companies can afford to lose or absorb the cost of replacing an experienced member of staff, yet all too many fail to understand and protect their crucial female employees' aspirations, says Croner.

There are also worrying indications that women who leave jobs in business, banking or finance will never to return to their former employer.

Richard Smith, HR expert at Croner, said: "Status and salary are not necessarily key drivers for women once they reach a certain level, but feeling valued and intellectually fulfilled is. It is important that employers consult with all their senior executives, listen to what they have to say, understand what motivates them and what they are looking to get out of their role."

Lynne Richmond was a communications professional who became increasingly dissatisfied with corporate life, and in particular dealing with "male egos and politics" at the expense of a more cooperative, common interest approach.

In 1998, she became selfemployed, and found that, although this was successful and financially rewarding, it did not fulfil her personal development goals.

Her interest in corporate social responsibility led her to take a related Masters degree at Bath University.

She said: "I originally enrolled on the course thinking it would lead to a change of direction for my consulting career but the degree changed the way I saw the world and gave me a desire to make a more direct contribution to issues relating to sustainability."

As female graduates now enter the business world in nearly equal numbers to their male counterparts, it is increasingly disconcerting that they drop out as they rise up the ranks.

How to retain this talent and increase women's representation in senior roles needs to become a serious concern for many employers, believes Croner.