IT comes as no surprise to many high-powered businesswomen that their sisters are turning their backs on senior management positions in several major British companies.

Tell them that the drop is 40% over the last five years, however, and there's likely to be an audibly shocked intake of breath, followed by a list of the hostile elements of corporate culture.

That is one of the main factors driving more women to set up their own businesses: the number of self-employed women in the UK has recently risen above one million for the first time.

"I am the classic example of this phenomenon," says Caroline Whitfield, the founder of Blackwood Distillers, the first distillery on Shetland. She was a corporate high-flyer before joining the "growing group of women saying stuff this for a game of soldiers".

With a degree from Oxford, an MBA from INSEAD, the European business school, and a career in the corporate world which included Coopers & Lybrand, Unilever and Hasbro, the toy company, where she was head of European business, she left to set up her own consultancy, mainly because she decided: "I needed to be true to myself."

For Whitfield, the last straw in corporate life was when a human resources executive asked her why she didn't go to do a doctorate on social anthropology.

"I thought: Did I go to sleep on the day when all the female chief executives were appointed?' "My career was, to some extent, outwith my control, and I didn't go through all that education, career dynamic and climbing ladders to have my destiny decided by something I couldn't influence. So I decided that I was going to do something for myself," adds Whitfield, who now thinks that women may be more driven to doing that exact same thing. When she raises the subject of starting their own business with men, they talk about the risks involved.

She now says of the corporate world: "I enjoyed what I did immensely, and I learned a huge amount. I think I was probably a bit much for some of those organisations. I am very energetic and driven. I feel that I must have a direct connection between my capability, output and reward, and it's basically tough what everyone else thinks."

Her new business philosophy, which has won her an export award for the white spirits being produced by Blackwoods in advance of the whisky, is: "I think I can do this, so I am going to do it."

The downside is that despite having gained control of her destiny, work is more demanding than ever.

"I have three young children who would argue that I should have a regular job, because they see other people with mums and dads who work nine to five," she says, but immediately counters that with the fact that company people in a senior position can't do that anyway. And then there's the stress, she says quickly, before having to switch off her phone as her plane is about to take off. "If you're in the middle of an organisation and you are answerable to the very top, the stress is unbelievable.

"Now I look at my MBA class and realise that hardly any women have stayed in the corporate world, and I don't think it is because they are making a lifestyle choice once they have children. People work much harder and have longer days as entrepreneurs."

A report published yesterday by Pricewaterhouse- Coopers spelled out the scale of women's flight from the corporate boardroom. It revealed senior management posts filled by women in Britain's top 350 quoted companies has dropped from 38% in 2002 to 22%. Sarah Churchman, head of diversity at PwC, pinpoints a trend which suggests that there will be fewer women in the very top positions in the future.

"Businesses tend to pay more attention to gender issues in senior positions and there appears to be an assumption that a supply from the middle ranks will eventually feed through. For big companies at least, this pipeline is shrinking at a worrying rate," she says.

In Scotland, that imbalance is clear not only in blue-chip companies, but also across the public sector. Although we congratulate ourselves on the number of women in the Scottish Parliament, there would have to be another 14 women MSPs to achieve equality.

To shatter the glass ceiling completely, according to the Equal Opportunities Commission in Scotland, we would need 111 more women in public appointments: 105 as secondary school heads, 21 MPs, 16 senior police officers, 13 judges, 12 local authority chief executives, 11 trade union general secretaries, 10 council leaders and 10 principals of further education colleges.

The EOC has also calculated that at the current rate of progress, across Britain it will take another 60 years to achieve an equal number of female directors at FTSE 100 companies.

It's not a lack of ambition. Many women - particularly mothers - are making the choice to become entrepreneurs and fitting the longer hours round the rest of their lives.

Jane Corbett was a manager for the Bank of Scotland sales team in Glasgow until her son was born 21 months ago. When she was due to return to work after maternity leave, she asked about working part-time, but it was clear that if she wanted her old job back, it would only be full-time.

The decision was further complicated because the family had moved house to Bridge of Allan, partly because of her husband's job.

"The practicalities of travelling and additional childcare would just have been horrendous if I had been working full-time, but part-time work would have been at a lower level," she says.

Instead, she resigned and with her husband, Iain, set up Bridge Asset Finance almost a year ago, to provide finance broking to small and medium-sized businesses. It has allowed her to make use of her skills, but also deal with domestic issues if necessary. "My son has had a recurring ear infection and he was sick last week, so I did have the flexibility for that, but employees are rarely offered the flexibility to come in late or not come in but make up the time in other ways, " she says.

Having been a manager, she does have some sympathy for employers, conceding: "If you are an employer, you want to know that your workers will turn up on time and will be totally dedicated to the job, as they would be if they did not have a family.

"I can see where employers have a problem with senior roles, but often people below management level find that the amount of flexibility they are given depends on their individual line managers."

At a senior management level, however, she believes the corporate culture is the culprit, and the main factor prompting women to work for themselves. Corbett says: "I know other mothers who have had fairly high-profile careers - some of whom are now running businesses from home and some are trying to work part-time and run a business. One has gone back on a part-time basis, but it has been reviewed after a year and there is now pressure on her to go back full-time, with the threat of redundancy if she does not.

"Although they agree that the arrangement is working, many of them say that they are worried about having to extend part-time working to other women, so it's definitely the corporate culture that is preventing her getting the flexibility that she needs."