When Dervilla Mitchell graduated with a degree in civil engineering, she was sent to a building site to give an assessment. It didn't take her long to realise there was a serious fault. She approached the foreman and told him matter-of-factly that a reinforcement was "incorrectly placed".

To her shock, the foreman picked up a giant steel bar, raised it over her head and shouted: "I've been in this industry for 30 years. I will not have a whippersnapper like you tell me how to do my job!"

Mitchell, 49, went on to become one of the UK's top engineers. The foreman went on to be fired. "Most male engineers tend to be quite benevolent to me," says Mitchell, who is now in charge of design management for the Terminal 5 building at Heathrow.

After 20 years, Mitchell still finds herself the only woman in most of her business meetings. And she has felt obliged to work full-time throughout her career, even with three children. "I didn't feel confident that I would succeed in my career if I worked part-time."

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Mitchell is one of a small band of female engineers who have climbed to the upper ranks of her profession. She was one of just four women in a year group of 200 men at university and has built a high-flying career for herself within a profession dominated by men.

Other women have been less successful. Only 25.2 per cent of the UK's science, engineering and technology workforce are women. And while women in science do well at undergraduate level, they do not thrive at professional levels. By the time they reach Mitchell's age, most women have dropped out. For example, in science, just 8.2 per cent of professorships are held by women.

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Lawrence Summers, former president of Harvard University, had his own theory as to why there are so few female scientists. In his infamous speech at the National Bureau of Economics in January 2005, he allegedly observed that women were intrinsically less able to do sciences. What he actually said was that there are fewer women than men in science partly due to innate aptitude - but that aptitude can be affected by socialisation and discrimination. By the time Harvard released a transcript of the speech, the PR disaster was unstoppable. Summers resigned his post in 2006.

A number of (mostly male) scientists have stoked the debate over the issue of women in science, notably Simon Baron-Cohen, professor of psychiatry and psychology at the University of Cambridge.

In The Essential Difference: The Truth about the Male and Female Brain (2003), Baron-Cohen argues that women's brains are physiologically different from male brains, the implication being that they are therefore less geared towards science. He identifies five brain types: "male", "female", "extreme male" (autistic), "extreme female" and "balanced". He says people with a "female" brain have well-developed empathy skills and so "make the most wonderful carers, teachers and nurses". Those with a "male" brain are good at systemising and so make excellent scientists, engineers and mechanics.

"This all sounds horribly familiar - not a big step from 'a woman's place is in the home'," says Phillipa Browning, a reader in physics and astronomy at the University of Manchester and a member of the Women in Physics committee at the Institute of Physics.

Although Browning acknowledges Baron-Cohen's point that some women have "male" brains and some men have "female" ones, "this seems almost pejorative, suggesting that a female engineer is not a real woman or a father caring for his children is not a real man".

The bottom line for Browning is that such research is not helpful and "may be used as an excuse by those who want to maintain barriers to women's progression. They can say that it doesn't matter that girls tend not to choose physics degrees or that women are rarely promoted to chairs in science - because we've all got the wrong sort of brain," she says.

The real reason that there are not more women in science, says Browning, is that "girls at school are still told that physics is too hard for them or they are reluctant to join classes full of boys". Once they graduate, female scientists "struggle to advance their careers, while their motility is restricted by being obliged to follow their partner's place of work". In addition, appointment committees tend to favour male candidates. "Let's not allow a piece of scientific research to distract efforts from solving these problems," she concludes.

Wendy Hall, professor of computer science at the University of Southampton, agrees: "The brain debate is irrelevant. It is a red herring and dangerous. As a society, we don't want people to have the same skills. Not every scientist can work as an island. Women are good at working in a team and organising. We all need each other. You don't need to have a mathematical brain to go into science."

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Hall says science needs a "PR lift" to attract more women. "Girls need to understand that science is exciting. Science created the atom bomb, it built fossil-fuelled power stations - but it also addresses climate, health and comes up with new drugs that tackle cancer."

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But perhaps more urgent is the need for culture change. "Scientific institutions are dominated by portraits of men. If you walk into a room of men, it can be very intimidating. It is easy to see why some women decide they have better things to do with their lives than struggle within a male-dominated culture," Hall says.

Hall tried to draw attention to gender-unfriendly language in the laboratory, but her efforts were not well received. "I got shouted down. Since then I've forced myself to pull back from talking about gender issues and do my stuff so I can say I've done it on merit."

Esther Haines, from the Women in Science, Engineering and Technology Initiative at Cambridge, says some men are not conscious of how their behaviour affects their female colleagues. "Women need to be more aware of how they present themselves. They need to work out the difference between being aggressive and standing up for themselves."

It would also help female scientists if their male colleagues were a little more welcoming, Haines says. "I remember going to a scientific talk called 'Britney Spears' Guide to Semiconductors'. The facts and figures had been superimposed on to a picture of Britney. I didn't say anything at the time. I didn't want to stand out. But women can feel uncomfortable with the style of committee meetings and the old boys' club atmosphere."

Efforts are being made to improve the situation. The Royal Academy of Engineering recently released a report aimed at increasing the number of women in the information technology and engineering workforce. "Employers are particularly looking for people with 'hybrid' skills - behavioural as well as technical - and there is a shortage of people with these skills," according to the report.

At present, only 20 per cent of people in IT are women, and even those are deemed "at risk" because they may leave work when they have children. Effort needs to be put in to allowing both men and women flexible hours, and money needs to be put into "career training" for women returning from maternity leave so they can stay on top of the latest developments in the field, the report says.

The UK Resource Centre for Women in Science, Engineering and Technology is doing its best to encourage universities to be more female-friendly through such initiatives as the annual Athena Swan award for those institutions that have made substantial progress in recruiting, retaining and advancing women. Measures might include allowing part-time working, positive action on promotion and courses to encourage girls into science.

Positive discrimination is not the answer, Mitchell says. "Much damage can be done by putting women in the wrong positions. Changes should not just be for women. Giving men and women the ability to choose how to run their working week helps everyone."

Still, progress has been achieved for women, she says. "When I was pregnant I was taken aside one day and told that I was 'a severe embarrassment to the section'. That would never happen now."

Susannah Spelling agrees. She is a postdoctorate research fellow in materials science at the Royal Academy of Engineering, and, at the age of 29, she has two children as well as an Excellence in Teaching award from the University of Oxford.

"Since I've graduated, I've never been made to feel special for being a woman. Because I have two children I work part-time, and the fellowship and university are very flexible. I have faced no opposition at all," she says.

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NOT JUST A BOYS' ZONE: A SNAPSHOT OF UK WOMEN IN SCIENCE

Women as % of total in UK chemistry 2005-06

GCSE: 42.2%

A levels: 48.9%

Undergraduates: 43.5%

Postgraduates: 39.0%

Researchers: 28.6%

Lecturers: 19.7%

Senior lecturers: 12.4%

Professors: 6.7%

Women in UK biology 2005-06

GCSE: 42.7%

A levels: 58.8%

Undergraduates: 60.2%

Postgraduates: 53.2%

Researchers: 46.4%

Lecturers: 32.9%

Senior lecturers: 24.2%

Professors: 12.6%

Women in UK engineering 2005-06

GCSE: n/a

A levels: n/a

Undergraduates: 15.4%

Postgraduates: 20.0%

Researchers: 18.7%

Lecturers: 19.1%

Senior lecturers: 12.9%

Professors: 5.7%

Women in UK computing 2005-06

GCSE: 47.8%

A levels: 36.3%

Undergraduates: 15.3%

Postgraduates: 18.8%

Researchers: 20.0%

Lecturers: 24.8%

Senior lecturers: 19.2%

Professors: 14.1%

Women in UK mathematics 2005-06

GCSE: 50.4%

A levels: 38.7%

Undergraduates: 39.3%

Postgraduates: 25.8%

Researchers: 17.3%

Lecturers: 20.7%

Senior lecturers: 14.9%

Professors: 6.3%

Women in UK physics 2005-06

GCSE: 40.8%

A levels: 21.4%

Undergraduates: 20.2%

Postgraduates: 25.0%

Researchers: 15.7%

Lecturers: 16.2%

Senior lecturers: 8.2%

Professors: 3.9%

Women as % of total in science, engineering and technology occupations by country 2006

Austria: 14.1%

Italy: 18.6%

Germany: 19.1%

Belgium: 19.6%

Netherlands: 20.5%

Spain: 21.4%

France: 22.4%

EU average: 22.9%

UK: 25.2%

Sweden: 25.3%

Poland: 28.2%

Ireland: 29.6%

Bulgaria: 33.5%

Lithuania: 38.0%

Latvia: 42.1%