Information services firm Experian studied 2.7m businesses and found the total number of female directors has increased by 240,000 since 2007.
However most of the increase can be attributed to progress at small businesses, while companies employing more than 250 people remain more male-dominated.
The news came as two women were promoted to senior positions on the boards of FTSE 100 firms. Jean Tomlin, currently HR director at Olympics organiser Locog, will join supermarket group J Sainsbury as a non-executive director after running the successful Gamesmaker volunteer programme.
Meanwhile, senior Credit Suisse banker Gillian Sheldon is set to take up the position of senior independent director at outsourcing firm Capita, where she is already a non-executive director.
Last month the EU failed to impose a 40 per cent quota for female representation on boards at listed companies.