CMS and Shoosmiths have become the first UK top 50 law firms to publish details of their gender pay gaps, ahead of an April reporting deadline for large companies.

The pair have joined more than 500 employers including easyjet, the Bank of England and PwC in publishing the data, which is required as part of new reporting regulations introduced by the UK Government last year.

All companies with more than 250 employees must provide the data by 6 April, including law firms, though they are not required to provide data about partners.

At CMS, the average UK hourly rate for women is 17.3% lower than men, marginally better than the UK-wide average of 18%. However, the firm's median average hourly rate for women – a figure which is less skewed by particularly high or low earners – is 32.8% lower than men.

CMS, which has a 70:30 split between women and men in its UK employee ranks, said these figures are "heavily impacted by the disproportionate female to male ratio in the firm, particularly in business support teams, as well as the high numbers of part-time female workers".

The information published on the government gender pay gap reporting site also includes data about bonuses, which shows bonus pay for women at CMS is on average 26.9% lower than men, a result the firm admitted was "disappointing". However, 90% of female UK staff received a bonus, compared with 85% of male staff.

In addition to the figures published on the government website, CMS has also released a more detailed analysis of UK gender pay breakdown at the firm.

The firm's report shows that for fee earners, there is little difference between average male and female pay, with female London lawyers actually paid on average 1.4% better than male lawyers. However, when looking at just senior associates in London, men are paid on average 0.22% better than women.

The firm's male lawyers in Scotland are paid on average 0.7% more than women, while in Bristol the pay gap is slightly more pronounced, with average pay for male fee earners 2.2% ahead of women.

CMS has also provided details about differences in male and female working patterns, with just 2% of male staff working part-time compared with 22% of women.

The firm said in its report: "Whilst our figures may well be in line with those of other international law firms, we are not complacent and accept that there is still work to be done.

"We are implementing an inclusion strategy that has embedded a number of initiatives within our firm that work together in a concerted effort to enhance the culture of our workplace."

At Shoosmiths, the pay gap is smaller, with the average hourly rate for women 15.4% lower than men and the median figure standing at 13%.

For bonus pay, the difference in mean earnings is 18%, with no difference at the median. Some 94% of male employees received a bonus payment in the 12 months before the report was published, marginally ahead of the female figure of 92%.

Shoosmiths chief executive Claire Rowe said: "We are pleased that Shoosmiths' median pay gap stands below the national average but we recognise there is still more work to be done. Over the past 12 months we set up a Gender Equality Working Group, which reports directly to the board. The group was established in recognition of the fact that a series of actions need to be taken at a firm level to advance gender equality.

"It provides a sounding board to discuss issues and potential solutions. Likewise, the board brings ideas to the group to collaboratively pin down the steps to be taken, giving our employees direct input and influence on decisions that will affect them."

CMS and Shoosmiths' pay gap data compares favourably with some of the other companies to have published their information to date.

At easyjet, the mean pay gap between men and women's hourly rate stands at nearly 52%, while at PwC, the average rate for women is 33% less than men. The gender pay gap for bonuses at PwC is even more pronounced, with women's bonus pay on average 59% lower than men.