Business secretary Vince Cable and home secretary Theresa May have written to FTSE 350 companies to remind them that they have just over a month to go to set targets for increasing the number of women in boardrooms.

In February, former trade minister Lord Mervyn Davies of Abersoch carried out an independent review, Women on Boards, which recommended that chairmen of FTSE 350 companies set goals for increasing the proportion of women in the boardrooms by September. He suggested a minimum figure of 25 percent.

In the letter to company chairmen, Cable and May said that there was a "strong business case" for increasing the proportion of women on boards.

"Inclusive and diverse boards are much more likely to be better boards, better able to understand their customers and stakeholders and benefit from fresh perspectives, new ideas, vigorous challenge and broad experience, which in turn lead to better decision-making," they wrote.

Some progress has been made since the report, Cable and May said, particularly among companies in the FTSE 100.

The number of all-male FTSE 100 boards fell from 21 in October 2010 to 14 this month, and in addition, they said that the rate at which women have been appointed to board positions has "significantly increased".

FTSE 350 companies that added women to their boardrooms this year include international distribution and outsourcing group Bunzl, which appointed Eugenia Ulasewicz as its first female director, and plumbing products firm Wolseley, which appointed Tessa Bamford as its first female board member.

However, Cable and May said that there was still more that could be done.

"We would encourage you to build on this by setting and developing targets and strategies, in order to effect the necessary change through means best suited to your own circumstances," the letter continued.

Lord Davies will be meeting with his steering board later this year to analyse the progress made on his recommendations, and will continue to report on the progress annually.

The reminder follows the recent launch of a Confederation of British Industry-backed (CBI)voluntary code drawn up by 19 executive search companies, which aims to encourage head-hunters to promote gender diversity at boardroom level.