Stada said on July 10 that Bain and Cinven would raise their offer by 25 cents per share to 66.25 euros after their previous 5.3 billion euro (4.68 billion pounds) bid fell through.

The private equity groups are also lowering their acceptance threshold to 63 percent, after the last offer secured the support of 65.52 percent of shareholders, short of the 67.5 percent they had targeted.

The newest offer is the latest twist in a takeover saga for Stada - one of the last remaining independent generic drugmakers in Europe - that has seen management upheaval and previous failed offers.

Stada's chief executive and its head of finance resigned this month after the failure of the most recent bid.

Bain and Cinven said on Wednesday they had already received irrevocable commitments to their sweetened bid from shareholders representing about 19.6 percent of Stada's stock.

Stada said its boards would examine the new offer document and issue a reasoned statement in a timely manner.

"As of today, both boards expect to be able to recommend the renewed offer for acceptance to the shareholders," it said.

Labour union IG BCE has meanwhile threatened to resist any further takeover attempts if Bain and Cinven's fresh offer fails.

"This is some high-level gambling, and Stada's employees will have to foot the bill later," Ralf Erkens, district chief at the union, said in a recent statement.

(Reporting by Maria Sheahan; Editing by Subhranshu Sahu and Louise Heavens)