U.K. contractors and design firms are reporting large gender pay gaps, among the biggest of all business sectors, according to new mandated reports submitted to the government by April 5.

Among select leading contractors, women's median hourly pay was, on average, 28% below men's, while the corresponding gap among design firms was 14%.

More than 10,000 U.K. companies in all sectors publicly filed the compensation data in compliance with a new law passed last April that required businesses with more than 250 employees to provide within 12 months a pay "snapshot" taken on April 5, 2017.

Among firms in all sectors, the pay gap averaged about 10%, according to the Financial Times. The gap within some financial service firms was as high as 59%.

Required data included median and average differences in hourly pay as well as the proportion of women in each wage-rate quartile.

Among contractors, the median hourly pay-rate gap reportedly ranged from 0.2% to over 48.5%. The proportion of women in the top quartile of pay rates ranged from 3% to nearly 17%. Women generally were among the lowest earners, accounting for up to 70% of the lower quartile.

The U.K.'s largest contractor, Balfour Beatty, reported a 33% median pay gap, while Bechtel U.K. recorded a 31.3% gap. French-owned Vinci Construction scored one of the largest pay gaps, at 48.5%.

Among the sample of design firms, the median gap was in the 11%-33% range and the proportion of women in the top quartile varied from 6.2% to 22%. The lower quartile was 28.5% to 54% populated by women.

Of the still-independent design firms, Arup reported one of the smallest gaps, at 16.7%, while Jacobs U.K. came in at 27.8%.

While the data are limited, they show that for all business sectors, the pay gap has shrunk only 3% to 20% in the last 15 years, while remaining unchanged among university graduates, according to the independent Institute for Fiscal Studies.

According to the research, "some, but not all, of the gender wage gap is explained by the fact that after having children women are much more likely than men to work part-time, and part-time work is associated with little or no subsequent progression in hourly wages."

Data on individual firms can be accessed here.