The Equal Opportunities Commission (EOC) has agreed to support a Goodman Derrick associate's sex discrimination claim against the firm for failing to make her a partner.
A spokesman for the EOC confirmed this week that backing has been given to the action, which Goodman Derrick is defending.
The proceedings, which were issued last year, are being brought by Claire Posner, who still works in the litigation department.
Posner, who is married with children and has been with the firm for eight years, claims the firm discriminated against her by promoting a male colleague to salaried partnership.
Since filing the discrimination claim, she has added a further claim of victimisation against her firm.
It is based on the way in which Goodman Derrick is alleged to have conducted its investigation and treated her since the complaints were raised.
The backing of the EOC has raised the stakes in the action, which has attracted widespread interest in the legal world.
It is believed to be the first contested case of sex discrimination brought against a law firm on these grounds.
A race discrimination claim filed by barrister Joy Okoye, against her old set Staple Inn Chambers in 1996, failed to get off the ground.
Wyn Lewis, senior employment partner at Biddle, who is acting for Posner, said: "I am obviously very pleased that the EOC has come on board to support the claims. The case raises important issues for female lawyers who, statistics show, have long been denied parity in pay and partnership prospects in the legal profession."
Biddle is instructing Thomas Kibling of Cloisters chambers.
Goodman Derrick, which is representing itself in the proceedings, is instructing Patrick Elias QC of 11 King's Bench Walk.
The case is listed for a final hearing on 26 April.
Tom Flanagan, an employment partner at Stephenson Harwood, said the EOC's involvement in the case meant it wanted to establish a precedent.
He said: "The involvement of the EOC may make more firms look at their procedures for promoting to partnership more objectively to see if they are fair. In reality they should do this in any case."