A voluntary code of conduct for executive search firms introduced a year ago, in response to a review by Lord Mervyn Davies, pictured, calls for women to make up 30% of a long-list when candidates are considered for top jobs.

Women now make up 16.7% of board members of FTSE 100 companies, and 10.9% of FTSE 250 boards, up from 12.5% and 7.8% respectively in 2010.

Lord Davies said: "I welcome the continued efforts and collaborations of executive search firms and business groups to ensure that we see a sustainable and consistent change and that talent is recognised regardless of gender."

Some 34 recruitment agencies have signed up to the code.

Secretary of State for Business, Vince Cable, said: "Diverse boards are better boards: benefiting from fresh perspectives, talent, new ideas and broader experience which enables businesses to better reflect and respond to the needs of their customers.

"This is good for women, good for companies who need to be the best they can be in order to compete in today's tough global market place, and ultimately good for the UK economy as a whole.

"It is essential that executive search firms and chairmen continue to use the code to increase this rate of change."