Boardwatch has been tracking the appointment of women to UK boards following the publication of Lord Davies's report in 2011.
Its latest figures showed that 17.3% of directors are currently women, only a marginal increase from August 2012, when the figure was 17.2%.
The figures showed that 86 more board seats need to be filled to reach the 25% by 2015 target set by Lord Davies.
There are still seven FTSE 100 companies with all-male boards, despite business secretary Vince Cable urging them to take action to increase the number of women in the boardroom earlier this year.
The figures did show that 34% of blue-chip board appointments in the last year have been women.
Overall female representation in the boardroom is up from 12.5% in 2010, before the Government made boardroom balance a priority.
The FTSE 100 has lost several high-profile female leaders in the last year, including Anglo American chief executive Cynthia Carroll and Pearson's Marjorie Scardino.
Jane Scott, who compiles the BoardWatch figures, said: "Overall 2012 showed good progress with FTSE 100 board positions rising from 15% to 17.4% and 27 companies achieving 25% women directors or above. But the very latest data reveals a plateau in the last quarter."
