Some 30 firms that are signing up to the Law Society’s new diversity charter will have to publish detailed statistics on diversity for the first time.
The scheme, which is being launched on 8 July, will require signatories to release figures annually on the age, gender, sexuality, ethnic backgrounds and religions of employees.
A number of FTSE100 companies, including BT, have also backed the charter and will ask all their legal suppliers to fill in a questionnaire on the issue.
Few law firms publish detailed statistics on the diversity of staff, particularly on sexuality and religion.
It is illegal in some countries to even ask employees about their sexual orientation, although the Law Society scheme covers only UK staff.
Ashurst head of diversity Helen Burton said the charter would put pressure on those who did not release data, adding: “If you look at what everybody else is doing, it might make you up the ante. Nobody wants to lose good lawyers.”