Norton Rose Fulbright, now a defendant in a $100m gender discrimination lawsuit that was originally filed against merger partner Chadbourne & Parke, is facing new claims that it aided and abetted Chadbourne's conduct by encouraging the firm to fire named plaintiff Kerrie Campbell before the merger deal closed.

A newly amended complaint also adds claims by a third plaintiff, Mary Yelenick, that Chadbourne pressured female partners to sign a letter disavowing the lawsuit.

The federal gender discrimination suit was filed about a year ago in the Southern District of New York, alleging Chadbourne's "all-male dictatorship" paid women partners less.

Along with Campbell – an ex-litigation partner who was the first to bring claims in the case – the firms' accusers include former Kiev office managing partner Jaroslawa Zelinsky Johnson and Mary Yelenick, a former chair of Chadbourne's products liability group who is now working as of counsel at Norton Rose. Yelenick sought in February to join the proposed class action as a plaintiff.

The amended complaint formally adds Yelenick as a plaintiff and names as a defendant Norton Rose Fulbright, which merged with Chadbourne on 30 June.

In the plaintiffs' first allegations levelled specifically at Norton Rose Fulbright, the new complaint claims the global Swiss verein aided and abetted Chadbourne's retaliatory and discriminatory conduct by "communicating to Chadbourne that it should formally expel Campbell from the firm" before the merger was sealed.

Chadbourne ousted Campbell in April, two months after the two firms confirmed they were planning to merge. Campbell has now opened her own practice.

"In fact, the transaction was not announced as finalised until after Chadbourne publicly announced that the firm would, and in fact did, expel Campbell," the suit said.

The plaintiffs allege that many members of Chadbourne's management committee now hold leadership positions at Norton Rose Fulbright, including Andrew Giaccia (US management committee vice-chair), Howard Seife (global head of bankruptcy, financial restructuring and insolvency), Ayse Yuksel (Istanbul partner-in-charge) and Abbe Lowell (US co-head of regulations, investigations, securities and compliance).

The amended complaint adds allegations by Yelenick, echoing her claims in an earlier declaration that the firm pushed her and other female partners to sign a letter last September criticising Campbell's lawyer, David Sanford.

Chadbourne has strongly denied the lawsuit's allegations. A Chadbourne spokesman on Thursday said: "No one in firm leadership pressured Mary Yelenick or any other female partner to sign the letter."

The amended complaint further includes Yelenick's concerns about Chadbourne's system for allocating origination credits for client work.

Yelenick claims the firm retaliated against her after she joined the case in February, by issuing a statement to the press that her record and accomplishments were exaggerated and that her claims were self-serving and baseless. "This characterisation starkly contrasted with the firm's strong praise of Yelenick's career and performance in a memorandum issued by defendant Giaccia upon Yelenick's retirement" from partnership last December, the suit says.

The defence lawyer in the discrimination case is Kathleen McKenna, a partner at Proskauer Rose, which itself is facing a parallel gender bias suit involving a Washington DC Proskauer partner and the same plaintiffs attorney in Campbell's case – Sanford.

McKenna and Norton Rose representatives did not immediately respond to requests for comment.