The UK's business secretary has launched a new code of conduct for head hunters in a bid to support more female appointments to FTSE 350 boards.

The Enhanced Voluntary Code of Conduct for Executive Search Firms comes after more than 70 firms signed up to the original voluntary code of conduct in 2011.

The new code is in response to the independent Sweeney review in February 2014, which called for the measure.

"Executive search firms are crucial to achieving gender diversity in both executive and non-executive roles," said Vince Cable, the business secretary.

"Recruiters can best show their commitment to this work by embracing this new enhanced code."

Under the new provisions, firms will provide evidence to the Davies Steering Group that they have, among other things, achieved a proportion of at least 33% female appointments across all their FTSE 100/250 board work.

The Enhanced Code also requires search firms to submit summary data in an agreed format to the independent Davies Steering Group annually, to confirm they meet the requirements for accreditation under the code.

Executive search firms will also be encouraged to visibly signal their commitment to gender diversity, on their websites and in discussion with clients as well as publishing relevant summary data on their track record.

The news comes after all FTSE 100 boards now have at least one woman serving on them, as the UK's top companies come close to meeting Lord Davies' 25% target for women on FTSE 100 boards by 2015.