Nearly a third of the UK's biggest companies largely rely on personal networks to identify new board members, the study by the Equality and Human Rights Commission (EHRC) found.
Most roles are not advertised, it said.
"Our top boards still remain blatantly male and white," said EHRC commissioner Laura Carstensen.
The study, which looked at appointment practices in the UK's largest 350 listed firms, which make up the FTSE 100 and FTSE 250, found more than 60% had not met a voluntary target of 25% female board members.
'Masking the reality'
In fact in 2012-13 and 2013-14, the period of the study, fewer than half of companies increased their female board representation.
The EHRC said the problem was particularly acute for executive roles, where nearly three quarters of FTSE 100 companies and 90% of FTSE 250 companies had no female executives at all on their boards during the time covered by the study.
The research comes just months after a report found there were no longer any all male boards in the UK's FTSE 100 companies.
But the EHRC said the "headline progress" of Britain's biggest companies was "masking the reality".
"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 recruitment process to the boards of Britain's top companies remains shadowy and opaque and is acting as a barrier to unleashing female talent," added Ms Carstensen.
The EHRC said too few companies were setting targets or encouraging applications from women with job descriptions and were relying on vague terms such as "chemistry" and "fit" rather than clearly defined skills and experience.
Other findings in the report:
Men outnumber women in senior positions in the FTSE 350 by a ratio of around 4:1
Three quarters of FTSE 350 companies have two or fewer women on their boards
Nearly three quarters of FTSE 100 companies have no female executive director and 90% of the FTSE 250 have no female executive directors
Companies with no women on their boards are more than twice as likely to rely on personal networks to fill roles than companies with more women on their boards
On Tuesday a government-commissioned review, led by Virgin Money's chief executive, Jayne-Anne Gadhia recommended that financial services companies link parts of executive remuneration packages to gender balance targets.
It also suggested that companies set internal targets for gender diversity in their senior ranks and publish progress reports.
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