The government's equality watchdog, set up to combat all forms of prejudice at work, faces a sex discrimination lawsuit this week from a female member of its own staff.

The case is a fresh embarrassment for Trevor Phillips, the chair of the Equality and Human Rights Commission, who is widely expected to be forced to step down when his contract ends in autumn following a string of internal disputes and allegations of financial irregularities. Several of his fellow commissioners have indicated they are likely to stand down if Phillips, who is close to several senior Labour politicians, including Lord Mandelson, is reappointed for another term.

The sex discrimination case brought by Brid Johal, an aide to Phillips's political adviser Faz Hakim, is scheduled to be heard at an employment tribunal in south London this week. It is understood her case centres on allegations that she was not told of a promotion that came up while she was on maternity leave and was therefore unfairly treated compared to other staff.