The UK's 100 largest law firms by revenue have seen their slowest income growth rate for six quarters, with the largest firms seeing the most sluggish growth.

Results from Deloitte's quarterly law firm survey show an average increase in fee income of 3.8% across the top 100 in the three-month period ending 31 July, compared with the same period last year. 

This is the slowest year-on-year quarterly growth since the period ending 31 January 2011. 

The top 10 UK firms saw the smallest increase in fee income over the period, posting a 1.7% rise, compared with 2% for firms ranked 11 to 25 and 4.5% for those between 26 and 50.

Deloitte professional services partner Jeremy Black (pictured) commented: "There isn't much M&A activity, which is probably why smaller firms have been performing better – the [lower-ranked firms] cover areas of work that are more mixed, while the larger firms that focus more on transactional work have taken a hit."

The research found firms in the 11-25 bracket saw the biggest increase in fee-earner headcount at 5%, compared with 1.4% for the top 10 firms and static numbers for firms from 26-50. 

The same group also saw the largest rise in chargeable hours, logging a 3.6% increase, however fees per fee earner dropped 2.5% suggesting an increase in pricing pressure.

The research also found that fee income is expected to grow by 5% over the top 100 for the year to the end of April 2013, with firms ranked 11-25 expecting rises of 7.1% – the highest forecast of the group. This contrasts with predicted growth of 3.9% across the top 10. 

Black added: "The larger firms have been getting a bit warier in their forecasts. On the other hand, most of the firms ranked 11 to 25 have distorted predictions because of recent merger activity. If the impact of it is stripped out, there is probably about 3% growth forecasted."

Simmons & Simmons senior partner Colin Passmore said: "The market remains tough and it's a comfort to be reminded that everyone faces the same challenges to win work. The fact that larger firms are growing the least isn't too surprising – the bigger the firm, the more mouths to feed."