An old boys' network is still in place in company boardrooms, holding back the number of women in senior posts, according to a new report.
A study by the Equality and Human Rights Commission (EHRC) showed that headline progress in achieving a target of 25% of company boards made up of women was "masking" the reality.
Three out of five firms in the FTSE 350 were failing to meet the target and fewer than half increased female board representation in recent years.
A third of companies relied on the personal networks of current board members to find new candidates, so the old boys' network was still being widely used, said the report.
The diversity of candidates was also being limited by virtually no open advertising of board positions, said the Commission.
Progress has been made on the 25% target for top firms set last year by Lord Davies in his Government-backed report into gender diversity in boardrooms, but the Commission said there was "inexcusable and unacceptable" variation within companies.
The position was worse for executive posts, with almost three out of four FTSE 100 companies and 90% of FTSE 250 firms having no women in the positions.
Laura Carstensen, Equality and Human Rights Commissioner, said: "Despite welcome progress and vital work by Lord Davies, our top boards still remain blatantly male and white, with inexcusable and unacceptable discrepancies between companies.
"The good work of a forward thinking minority masks that many top businesses are still only paying lip service to improving the representation of women on boards.
"The best companies are showing that having talented women on their boards is boosting both performance and fairness.
"Unfortunately, the recruitment practices of too many businesses still remain trapped in permafrost and that's holding back women and ultimately the companies themselves.
"The recruitment process to the boards of Britain's top companies remains shadowy and opaque and is acting as a barrier to unleashing female talent."
The commission has made a series of recommendations, including stopping the use of personal networks to make appointments.
TUC general secretary Frances O'Grady said: "These figures make for depressing reading.
"Despite the hard work of Lord Davies, it is clear that voluntary measures have not led to meaningful change in company boardrooms.
"The only way we will end the old boys' network is if ministers act to ensure all board posts are advertised publicly and move towards compulsory quotas for women. Without this, female board members will continue to be a rarity."
Sam Smethers, chief executive of the Fawcett Society, said: "It is unacceptable that almost half of FTSE 100 companies and over half of FTSE 350s have not achieved the 25% target.
"This is extremely worrying and suggests the more stretching target of 33% will require an additional intervention.
"In our view the time-limited use of quotas needs to be part of the solution.
"International evidence suggests that this approach works. What we need is more representative and more effective boards. It's time to speed up the pace of change. The voluntary approach just isn't going to work for many of these companies."