Thermo Fisher in mid-July raised its bid for Qiagen by 900 million euros (813.07 million pounds) to 11.3 billion euros but failed to win over Davidson Kempner, a key investor in the German company.

The hedge fund on Friday said Thermo Fisher was free to relaunch the offer immediately with Qiagen's approval if it fails to reach the shareholder acceptance threshold on Aug. 10.

It added that Qiagen's non-executive Chairman Hakan Bjoerklund and the board may not be clear about this.

"Following our call with Dr. Bjoerklund on 30 July 2020, we are deeply concerned that there is a misunderstanding of the German tender rules amongst the board and possibly amongst shareholders, particularly with regards to the 12-month blocking period applicable after an offer fails," the hedge fund said in an open letter to Qiagen directors and top executives.

As part of the sweetened offer, Thermo Fisher reduced the minimum acceptance threshold to 66.67% of the target's share capital from 75% and extended the deadline to vote on the deal by two weeks until Aug. 10.

Qiagen was not immediately available to comment.

(Reporting by Ludwig Burger; Editing by David Evans and David Holmes)