An unnamed partner in Proskauer's Washington DC office has sued the firm in federal court, alleging she is a victim of discrimination and claiming "substantial gender disparities" in the firm's partnership. The suit claims at least $50m (£39m) in damages.

Proskauer called the claims "groundless" and suggested that the partner sought to force a payout after her practice faltered.

The complaint was filed on Friday (12 May) by lawyers at Sanford Heisler Sharp – which is also leading a high profile gender bias lawsuit in New York on behalf of current and former female Chadbourne & Parke partners. Proskauer represents Chadbourne in that case.

The plaintiff in Friday's lawsuit accuses Proskauer of paying her millions of dollars less than her male counterparts, despite her "standout performance" at the firm.

"Among other things, Proskauer excluded plaintiff from client matters, declined to allow plaintiff to pitch or to participate in any employment litigation matter for firm clients, rebuffed her efforts to assume a greater leadership role at the firm, tolerated and facilitated an environment where she was targeted for harassment and humiliation by firm leadership, demeaned and belittled her to her peers and clients, and refused to rectify pay disparities," the suit alleges.

In a separate filing on Friday, Sanford Heisler argued that court should keep the plaintiff – identified as "Jane Doe" in the complaint – from being named to protect her personal information.

Proskauer's DC office only counts two women as partners, according to the firm's website. They are labour and employment partner Connie Bertram, who is co-head of the firm's whistleblower and retaliation group and head of its government contractor compliance group; and Ann Ashton, a securities and white-collar defence partner. Neither of them responded to requests for comment on Friday.

Bertram joined Proskauer in 2013 from Cooley, while Ashton joined in 2012, around the time her former firm, Dewey & LeBoeuf, went bankrupt.

Proskauer said on Friday that the plaintiff had just completed her fourth year at the firm.

In addition to pay discrimination, the complaint alleges that the female partner "has been overtly objectified based on her sex". It claims Proskauer chairman Joseph Leccese has frequently commented on the partner's appearance, describing her with words like "elegant" and "glamorous." It also makes specific allegations about prominent securities partner Ralph Ferrara, saying he "has been far blunter" in his comments. (Like Ashton, Ferrara joined Proskauer in 2012 from Dewey & LeBoeuf.)

"Almost every time he saw plaintiff in the office, he made suggestive or inappropriate comments about her appearance. He drew close to her in a lascivious manner and made inappropriate comments regarding her appearance, body, clothing, or 'sexiness'," the suit alleges.

Neither Ferrara nor Leccese is named as a defendant in the suit. They did not immediately respond to requests for comment.

Proskauer has issued a strongly worded statement blasting the partner's claims. The firm said that the plaintiff – whom it also did not identify by name – was among the five most highly compensated partners in her department, and had "offered little complaint about her compensation until just a few months ago, when her business began to decline dramatically and she apparently became fearful that her compensation might be reduced".

The statement continued: "Instead of seeking to rebuild her practice, she sought to squeeze a massive payout from our firm in exchange for her rapid departure and an agreement not to weaponise her blatantly inaccurate charges. This action resulted only when we refused to capitulate to such questionable tactics."

David Sanford, a lead lawyer for the plaintiff, said that he did not have much to add to what was alleged in the complaint, and he declined to give the plaintiffs name.

"The complaint is rich with detail. Some of the detail is redacted and at some point down the road it will get unredacted," he said.

Sanford has a history of taking on Proskauer on at least two other fronts.

He is pitted against the firm in the similar Chadbourne gender bias case pending in New York, in which Chadbourne is being defended by Proskauer partner Kathleen McKenna. And in 2011, Sanford represented former longtime Proskauer CFO Elly Rosenthal in a lawsuit alleging that she was discriminated against because of her gender, age and a perceived disability. Rosenthal's suit settled in 2013.