Larger firms have less diverse cohorts of trainees than smaller ones, according to data obtained from the Solicitors Regulation Authority (SRA) by Legal Week.

The news comes as the judiciary has embarked on a mentoring programme for women, people from ethnic minorities and people from less advantaged backgrounds amid fears the bench is not representative of the diversity of modern Britain.

The SRA data shows that at firms with more than 81 regulated solicitors, the largest firms the regulator monitors, black and minority ethnic (BME) trainees accounted for 22.5% of all trainees in 2014 compared with an industry average of 29.6%.

At firms with between two and four regulated solicitors BME representation among trainees rose to 51.6%, while at firms with between five and 10 regulated solicitors 43.2% of trainees were BME.

The largest firms also had a lower proportion of female trainees. On average last year, 61.6% of all trainees regulated by the SRA were female. This rate dropped to 57.7% at firms with more than 81 regulated individuals.

At firms with between two and four regulated individuals 63.8% of trainees were female.

SRA executive director for strategy and planning Richard Collins said that following the regulator's risk outlook report in July 2014, which identified a lack of diversity in the profession as a key risk to regulatory compliance, the SRA was continuing with a programme of supervision visits to larger firms to discuss compliance with its equality and diversity principles.

Collins added that the SRA is also "committed to supporting firms to collect and publish their diversity data as this will help firms identify the gaps".

The largest firms have seen a marginal improvement in both ethnic minority representation and gender diversity since 2012. The proportion of ethnic minority trainees at the largest firms has increased from 17.5% to 22.5% in 2014, and the proportion of female trainees rose slightly from 56.7% in 2012 to 57.7% in 2014.

Founder of diversity organisation Aspiring Solicitors Chris White said: "From my experience over the last year the large firms are extremely keen on promoting diversity (in its broadest sense) by becoming either a founder or affiliate member of Aspiring Solicitors.

"It is fair to say that everyone recognises this is a marathon not a sprint and it will take years rather than months for our profession to be where it should be in terms of diversity (across all underrepresented groups)."

Across all firms, BME representation among trainees has risen by more than seven percentage points from 22.4% in 2012 to 29.6% in 2014.

The three largest UK partnerships by headcount - Clifford Chance, Allen & Overy and Freshfields Bruckhaus Deringer - were unavailable for comment.

This week the judiciary also announced a scheme to boost diversity.

Over the coming months the judiciary will run a mentoring and training programme for women, people from ethnic minorities and people from less advantaged backgrounds to try and prepare them to apply for positions on the bench.

The judiciary's mentoring scheme comes in response to concerns that the traditional route into the judiciary through becoming a top solicitor, barrister or general counsel is creating a barrier to underrepresented groups joining the bench.

Lady Justice Heather Hallett said: "We fear that we may be missing out on a pool of talent for whom the traditional route is not an option."

The initiative is designed to complement a new version of the Judicial Appointments Commission's recruitment process, which will be run for the first time this summer.

In July the Judicial Appointments Commission will recruit 14 deputy High Court judges, but for the first time the process will be open to lawyers who have not been Recorders in the court – a move designed to open up routes into the judiciary.

Hallett hopes that the two initiatives combined will "attract more exceptionally high-quality lawyers and legal academics from non-traditional backgrounds to sit in the High Court".