Stada's management is now reviewing the new offer, which is at the same price as the previous 5.3 billion euro (4.66 billion pounds) bid but lowers the shareholder approval threshold to somewhere below 65 percent, the sources said.

The private equity firms' previous offer of 66 euros a share, launched in April, secured the support of 65.52 percent of Stada's equity but failed to go through because the threshold was 67.5 percent.

Stada's management had backed the original takeover and Bain and Cinven will launch the new offer once the Stada board and Germany's financial watchdog have cleared it, the sources said. That could be as early as next week, they added.

The first bid missed the threshold despite the 49 percent premium offered by the buyout groups to trump a rival offer from private equity duo Advent and Permira.

Some shareholders, mainly hedge funds, held back some of their shares, speculating on securing a higher price for any remaining stock after a successful initial tender offer.

Bain and Cinven have set the acceptance threshold at below 65, although financing banks had given them room to reduce it to as low as 60 percent, the sources said.

The bidders obtained firm commitments for their new offer from significantly more than 15 percent of Stada's voting rights, giving them confidence that the offer will go through, the sources said.

Activist investor Elliott Management's move to buy a Stada stake has delayed the board's review of the offer, the people added.

"Stada wants to be extra cautious not to make any mistakes that could draw any litigation," one of the people said.

The size of the stake Elliott has bought remained unclear. It will have to be disclosed shortly, if the regulatory 3 percent or 5 percent thresholds have been crossed.

Elliott, which has not expressed a view publicly on the takeover bid, has not met with Bain and Cinven and has not made any specific proposals regarding the potential new bid, the sources said.

Stada's chief executive and its head of finance resigned on Tuesday ahead of the expected new bid, adding a note of uncertainty to the takeover process, which has proven to be more complex than the private equity firms anticipated after launching their blow-out offer.

Bain, Cinven, Stada and Elliott declined to comment.

(Additional reporting by Edward Taylor, Patricia Weiss; Editing by Greg Mahlich and Susan Fenton)

By Arno Schuetze and Alexander Hübner