The lack of women at the top of Britain's biggest companies is laid bare today as research by The Independent reveals the full extent of the gender imbalance in UK boardrooms.

When the former trade minister Lord (Mervyn) Davies was appointed to look into the problem last week, the coalition Government admitted that only 12 per cent (139) of the directors of FTSE 100 companies are female. But the situation is far worse.

Because several of them hold multiple positions, in fact there are only 120 individual women on boards, out of the total pool of 1,100 directors. And just 20 of them – 6 per cent – are the executive directors who run the company on a daily basis, compared with the 309 who are male.

Only five FTSE 100 companies have female chief executives: the asset managers Alliance Trust, publishers Pearson, miners Anglo American, fashion house Burberry and Imperial Tobacco. Burberry and Alliance Trust are also the most balanced overall, each with three women on their eight-strong boards.

But more than a fifth of the FTSE 100 companies have no female representation at board level whatsoever. In only three companies do women make up more than a third of directors. Not one has a gender split on the board of 50:50.

Pressure groups renewed calls for real action to address the stark inequality yesterday. "The tiny number of women at the top of business is totally unacceptable," said Nan Sloane, director of the Centre for Women and Democracy pressure group. "The general public has the feeling that the equality battle has somehow been won, but it is nowhere near won. While there's a lot of talk about improving the number of women in business, there is virtually no action at all."

There is little expectation that the latest review from Lord Davies, due to be presented to David Cameron in December, will prove any more effective. Despite decades of discussion, progress is glacial. In the 10 years since Cranfield University School of Management produced its first "Female FTSE" report, the number of female directorships has risen only 5 per cent.

And ingrained attitudes to the role of women remain intact. One headhunter contacted by The Independent yesterday said that female representation is higher in sectors where women "really matter", such as retail, while under-representation in "masculine" industries like engineering or defence is of much less concern.

Baroness Hogg, chairwoman of the Financial Reporting Council, said: "There is a huge amount of female executive talent in British industry, the problem is that it is not visible." The FRC recently added clauses to its corporate governance code underlining the importance of diversity, particularly in gender, for the first time. "The next two years are pretty important because it will be a test of intent," Baroness Hogg said.

The Equality and Human Rights Commission estimates that it could take three-quarters of a century before directorships are balanced equally between the genders. "Workplaces need to change if companies are to attract and retain the best employees," a spokeswoman said. "Long hours, a lack of flexible working options and direct discrimination remain some of the biggest barriers to addressing gender inequality in the workplace."

Elsewhere in Europe, the picture is very different. The leader is Norway, where at least 40 per cent of company directors are now women, thanks to laws introduced in 2006. The strategy is spreading fast. Spain has a 40 per cent quota, although it is not yet legally mandated. Italy, France, Finland, Germany and Sweden have all either set similar recommended levels, or have legislation going through parliament. And Europe's Fundamental Rights Commissioner, Viviane Reding, last month warned that unless more European boardroom seats are filled by women by 2011, she will impose a legal quota.

Quotas draw little support in Britain, and are unlikely to be included in Lord Davies's conclusions later this year. Detractors claim the Norwegian system over-promotes women who lack qualifications or are the wives or daughters of other directors.

Professor Susan Vinnicombe, the director of the International Centre for Women Leaders at Cranfield University, says the entire process of appointing board members is "flawed", operating "like a fiefdom" of the company chairman based on cliques and personal relationships. "It is a weird and mysterious exercise that is tilted away from all outsiders," said Professor Vinnicombe. "There are a whole lot of men who get overlooked as well, but the worst affected are women."

Campaigners reject the oft-repeated claim that there are simply not enough women qualified to take on top jobs, pointing out that there are more than 2,500 women on public companies' executive committees – one step below the board – let alone the thousands more flying high in private companies, the public sector and charities.

Such is the antipathy for overt regulation that insiders expect Lord Davies's recommendations will follow the Australian example, using the "soft power" of lobbying efforts and mentoring programmes rather than the blunt instrument of a quota.

So far, the results look better. In the first six months of 2010, 31 women have been appointed to major companies' boards, compared with just 10 in the whole of last year.

100 companies – only five female bosses

Cynthia Carroll - Anglo American

Boss who met barrage of sexism

After 19 years in the aluminium industry, Ms Carroll might have thought she had seen it all. But as the first female boss of mining giant Anglo American she was publicly accused of sexually-frustrated professional incompetence by no less than the company's former deputy chairman.

At the height of a long-running takeover battle with rival Xstrata last year, Graham Boustred told South Africa's Business Day newspaper: "This woman's hopeless."

But that was just the beginning of Mr Boustred's barrage of sexist prejudice. "Do you know why it's difficult to find a female CEO?" he said. "It's because most women are sexually frustrated. If you have a chief executive who's sexually frustrated she can't act properly." Notwithstanding Mr Boustred's analysis, Ms Carroll saw off Xstrata and is widely regarded as one of the mining industry's most successful bosses.

Katherine Garrett-Cox - Alliance Trust

Prodigy who combines work with four children

A star of the asset management industry, she is one of a handful of "superwomen" known for juggling a large family and a high-flying career.

Known as "Katherine the Great" among her peers, there was little surprise when Ms Garrett-Cox was bumped up to the top job at Alliance Trust in 2008, just 15 months after she joined the company. The businesswoman earned her nickname early in her two-decade career, when she was appointed head of US equities at Hill Samuel at the age of 26. She now leads a company founded in 1888 which manages several billion pounds-worth of assets.

Marjorie Scardino - Pearson

Still going, the FTSE 100’s first female chief

When she was appointed to run the vast Pearson publishing empire in 1997, she instantly became a symbol for women trying to crack the glass ceiling. American-born Ms Scardino – who was ranked the 19th most powerful woman in the world in 2009 – was the FTSE 100's first ever female chief executive and her appointment was widely hailed as a trailblazer for others. But it would be another eight years before another woman followed her into the big league – Dorothy Thompson, the boss of power station company Drax, which has subsequently dropped out of the FTSE 100.

Alison Cooper - Imperial Tobacco

She made it – and doesn’t see why others can’t

When she took up the post of chief executive in April, Ms Cooper was only the second English woman FTSE 100 boss. Unlike the majority of her female counterparts, Ms Cooper has been outspoken on the role of gender in business, roundly dismissing the concept of a glass ceiling limiting women's achievements. "I can't say I have ever seen a glass ceiling," she said shortly before taking up the top job. She is also against regulation to address the marked gender imbalance. "I am not in favour of any forced proportions of women on boards," she said.

Angela Ahrendts - Burberry

£1.8m bonus shows value of fashion label supremo