Step to one side Sarah Connor. Terminator Arnold Schwarzenegger has a new foe Labour's deputy leader Harriet Harman. Last month Harman appealed to California's governor to shut a US website that provides reviews of British prostitutes. Of course, no one had heard of this site until Harman kindly alerted the world to it but I am sure its creator is thanking her.

It's not the first time that the minister for women and equalities has taken on a mighty opponent. Back in August she took her diversity push to the City when she blamed all-male boards for the financial crisis. Things would have been different, she said, if it had been Lehmann Sisters rather than Lehmann Brothers.

Harman wants to improve Britain's economic performance by throwing out the "nightmare of men-only boards", as she told a Treasury Select Committee investigation into sexism in the City. Harman's argument that a "critical mass" of women at board level would improve financial performance may have men spluttering into their malt whiskeys, but evidence is on her side.

A Catalyst report, Linking Performance and Gender Balance on the Board, found a correlation between the number of women in a company's leadership and its profitability. Fortune 500 companies with three or more women on the board had a 73% return on sales, 83% return on equity and 112% return on invested capital compared with boards with the fewest, or no, women.

Meanwhile, according to Nick Wilson of Leeds University Business School, having women on the board can reduce a company's risk of bankruptcy by a fifth.

Yet, despite this, women make up just 11.7% of board directors, according to the Female FTSE Report 2008, and only 16 (10.7%) of the 149 new appointees to boards in the past year were women. Worse, nearly a quarter of FTSE 100 companies still have male-only boards.

Earlier this year, David Walker's review of corporate governance and financial institutions concluded that more diverse boards were vital for the long-term success of organisations. Meanwhile, last month boardroom consultancy firm IDDAS released the results of research into the experience of a fifth of all females currently on FTSE boards. The research found that women are so well qualified and prepared for a board role that, as one board member, says: "They raise the bar for everyone."

With last month's World Economic Forum ranking showing the UK has slipped down the international league table for gender equality, companies must act. For by next year just 25% of the UK's full-time workforce will be white able-bodied men under 45. Only those with a diverse boardroom make-up in the future will survive.n

Siân Harrington is editor of Human Resources magazine.