Only 12% of directors appointed in the two months to May were women, down from the 50% rate seen a year ago. This means just four of the 34 new directors were female.
The figures published by The Professional Boards Forum BoardWatch found another 84 more board seats held by women are needed to reach the 25% target set by Lord Davies in 2011.
The figures also showed the proportion of women on FTSE100 boards has been stuck at 17.4% since August, after rising from 12.5% in 2010.
There are still five companies with all-male boards: Antofagasta, Croda, Glencore Xstrata, Melrose and?Vedanta Resources.
Lyndsey Oliver, co-founder of gender balance consultancy Female Quotient, said despite all the focus and attention this issue has been getting, the "figures are disappointing". She also warned companies "can't allow progress to stall".
"While the benefits are proven, it seems we have some way to go to change mind-sets," Oliver said.
"Boards have to look beyond the safety of the old corporate stereotypes and consider more creative appointments. It is too easy to stick with established patterns of behaviour and simply look for candidates to fit the current mold."
She added: "It's tough to take a perceived risk, but risk has its rewards. The charity sector is one example where bold appointments have been made, recruiting outside of traditional backgrounds - and to great effect."
There was better news relating to FTSE250 companies, where figures showed 40% of all board appointments in the three months to May were female. The figures also revealed there are now 25% of companies with all-male boards, down from 52% in the previous three months.
In 2011, a report by Lord Davies recommended all FTSE100 companies should have 25% female representation on their boards. This was reiterated earlier this year, when business secretary Vince Cable wrote to the remaining companies with all-male boards, urging them to increase the number of women in their boardrooms.
Oliver said: "Despite the disappointing figures quotas are still not the answer. Instead, developing the talent pipeline will be key to not only hitting targets, but also more importantly sustaining these achievements.
"Hitting any target means nothing unless there is systemic and pervasive change at all levels. Diversity and inclusiveness must be embedded in every part of talent management and cultural change always starts from the top. If organisations start with inclusiveness, diversity will naturally follow."
