The UK’s private firms are falling behind those listed on the stock exchange when it comes to delivering gender equality in the workplace within the most senior positions.

According to research conducted by analyst BoardEx on behalf of the Observer newspaper, the number of women on boards at non-listed companies is generally far lower than among those on the FTSE 100 and 250.

The study found that out of Britain’s top 100 private companies, just 64 publish the composition of their boards, the newspaper reports.

Of those, 56 per cent possess boards staffed completely by men, while 73 per cent have all-male teams of executive directors.

In comparison, there are eight firms currently on the FTSE 100 with all-male boards, while 28 per cent of boardrooms of companies on the FTSE 250 are made up entirely of men.

Following the publication of the Davies report in 2011, which introduced a voluntary FTSE 100 target of 25 per cent of directors to be female by 2015, there has been reasonable progress in increasing gender diversity in the boardrooms of the UK’s largest listed firms.

Indeed, figures show that 17.7 per cent of FTSE 100 directors are women, up from 12.5 per cent at the time of the Davies report’s publication.

However, these new figures suggest that private firms are lagging some way behind their publicly listed peers.

Ann Francke, chief executive of the Chartered Management Institute, which has been one of the leading campaigners pushing for more female representation in the boardroom, described the data on private firms as “disappointing”.

She told the newspaper: “This underscores the endemic nature of the issue.”

However, a study by credit rating agency Experian published in December revealed that the number of women in director positions at UK firms has risen dramatically in recent years, from just over 993,000 five years ago to more than 1.23 million today – an increase of 24 per cent.