The U.K.'s Investment Association and the Hampton-Alexander Review called on 69 FTSE 350 companies to improve gender diversity on their boards.
The Investment Association is a trade body that represents U.K. money managers overseeing more than £7 trillion ($9.2 trillion), and the Hampton-Alexander Review is an independent review body that seeks to increase the number of women on FTSE 350 company boards.
The two groups have written to companies that have no women or just one woman on their board.
The letters ask the companies to outline what action they are taking to make progress and ensure they are meeting the Hampton-Alexander targets of 33% of women on their board by 2020.
"Investors have been consistently clear that they want to see greater diversity in the boardroom so it is totally unacceptable that 1 in 5 of the U.K.'s biggest companies are falling so far short," said Chris Cummings, CEO of the Investment Association, in a news release. "These companies must up their game and explain clearly how they are planning to meet the Hampton-Alexander targets, or risk investor dissent at their annual general meeting."
Sir Philip Hampton, chairman of the Hampton-Alexander Review, added in the news release: "There's a surprising number of boards with just one woman, which looks more like tokenism than diversity. It also does not reflect the population of very talented women capable of making great contributions in boardrooms."