More than two thirds of the UK's top 20 firms have now put in place gender diversity targets for partners or management committees, with Clyde & Co and Simmons & Simmons the latest to reveal their ambitions.

However, six of the top 20, including two magic circle firms, have either not put targets in place for female representation among the full partnership or on their executive committees, or would not reveal their plans. The firms are: Slaughter and May; Freshfields Bruckhaus Deringer; DLA Piper; Bird & Bird; DAC Beachcroft; and DWF.

Clydes has outlined plans to reach a 30% female partnership, with head of HR Charlie Keeling saying it is looking to achieve the percentage "as fast as we can". Women currently make up 22% of the firmwide partnership.

Earlier this week, Simmons revealed it was aiming to have 30% female partner promotions on a rolling basis.

The firms are among the 14 in the top 20 to have introduced identifiable partner targets, although Linklaters has set percentages only for representation on its executive committee and promotion rounds rather than its full partnership. Similarly, Irwin Mitchell is aiming for a proportion of 25% on its committees and boards by 2016.

Addleshaw Goddard has reached its goal of 20-25% female representation for 2014 (22%). A spokesman for the firm said the partnership is currently consulting on firmwide strategy, which is expected to include new "aspirations for diversity" when it is rolled out at the end of this year.

However the effectiveness of targets continues to be a topic of debate. A managing partner at a top 20 firm said: "We keep talking about diversity but most of our female partners actually seem opposed to [targets], which we can't ignore. On the other hand, progress on this front is slower than we'd like. Firms have been struggling with the issue, and some think targets will help, but I can't think [they] would make a revolutionary change."

Other firms to have outlined targets in recent months include Norton Rose Fulbright, which is aiming for women to account for 30% of its partnership by 2020, Berwin Leighton Paisner, which is targeting 30% by 2018, and Ashurst, which has set a 2018 target date for 25% of equity partners.