Fewer than a third of new partners appointed in Britain’s biggest law firms over the past three years are women, according to new research.

The number of female promotions was even lower in the “magic circle” firms, with women taking up only one in five new partnership roles in the City’s legal elite.

The study, which is due out this week, will add to pressure on top law firms to encourage more women into their senior ranks at a time of growing concern about the lack of diversity in the top levels of the profession.

Women make up less than 20 per cent of the total of partners in Britain’s 30 biggest law firms, the study, by Legal Week, a trade publication, will show. Among the magic circle firms — Allen & Overy, Clifford Chance, Freshfields Bruckhaus Deringer, Linklaters and Slaughter and May — the average is only 15 per cent.

This is despite women now making up more than 60 per cent of the graduate intake of many big firms.

The disparity between the percentage of women lawyers among their junior ranks and in their partnerships is seen by many law firm leaders as one of the biggest issues now facing their business.

Vast numbers of talented female lawyers choose to leave big firms in their early 30s, on the verge of becoming partners, after deciding that starting a family is inconsistent with the demands of commercial law at the top level. Senior partners admit that law firms have not done enough to offer alternative career structures to convince young women to stay after having families and fear that, if they cannot encourage more to become partners, firms will face a critical shortage of senior talent in coming years.

Despite growing awareness of the problem, only 28 per cent of partners appointed in the UK’s top 30 firms in the past three years are women, according to Legal Week’s research, to be published on Thursday. Over that period, only two firms — Shoosmiths and Irwin Mitchell — promoted as many women as men to partnerships.

Among the City’s ten biggest firms, women had the best chance of rising to partner at Herbert Smith (where 33 per cent of new partners were female), DLA Piper (28 per cent) and Eversheds (27 per cent). Women comprised only 15 per cent of promotions at Clifford Chance, Freshfields and Linklaters, the three biggest magic circle firms.

Last year, Clifford Chance made up three women out of seventeen partners worldwide. Freshfields promoted three women out of fourteen partners, and Linklaters made up two out of eighteen. Allen & Overy promoted the most women among the magic circle in 2009, making up eight new female partners out of a total of twenty.

David Childs, Clifford Chance’s managing partner, said: “The gender balance among our partners is still not where we want it to be and we recognise that there is no one answer that will solve a problem like this, which stems from a wide range of causes. We believe that what’s important is support and attention from top-level management and, for this reason, gender representation will continue to be a key focus of management.”

Clifford Chance said that its overall gender balance was a key consideration in deciding new promotions and it planned to double its number of female partners from 15 per cent of the worldwide total to 30 per cent. It has introduced measures such as a women’s network to encourage young women to become partners.

Allen & Overy said last month that it would let even top partners work part-time in order to make partnership more appealing to women. From May, even the firm’s most senior members will be able to work a four-day week for up to eight years.

Freshfields and Linklaters are introducing measures to make it easier for women to become partners, including mentoring programmes and improved maternity leave. Linklaters said that it had already had some success, increasing the percentage of women partners in its London office to 24 per cent, against to 15 per cent globally.

Alex Novarese, Legal Week’s editor, said that debate about diversity in the legal profession had been tarnished by “lip-service and jargon-loaded initiatives”, but welcomed changes to working practices by several big firms.

Five lead the field

Analysis: Alex Spence

Gone are the days when the top ranks of Britain’s biggest law firms were exclusively male, yet some firms have been better than others at removing the glass ceiling.

Over the past three years, women made up more than 40 per cent of new partners in only five firms in the top thirty: Shoosmiths; Irwin Mitchell; Berwin Leighton Paisner; Field Fisher Waterhouse; and Beachcroft. What are those firms doing to promote women that others are not?

Alison Eddy, a partner in Irwin Mitchell, said that it monitored the composition of its workforce, including checking annual appraisals and pay awards for any trace of bias. It has also set up groups led by senior female partners that provide a forum for women to discuss career issues.

Shoosmiths, which has the highest percentage of female partners, at 33 per cent, and is one of the few firms in the top 30 to be run by a woman, has no specific gender policy. Louise Hadland, head of human resources, said that its success at encouraging women into its partnership was simply the result of an open, inclusive culture.

Rachel Dineley, of Beachcroft, said: “It’s not about being male or female, it’s about a spirit of co-operation.” Beachcroft partners have led by example by adopting flexible working.

So far, regional firms have outdone their City counterparts at appointing women partners. However, the London elite focus on multinational deals that involve gruelling hours.

The head of a top City firm said that he had begged many of its brightest young women lawyers to stay in it, but once they wished to start a family they would put up with work travel and clients’ late-night calls no longer.

“Magic circle” firms know that in seeking continuity of talent, they are running out of time.