The U.S. laboratory equipment supplier said it had lifted its bid to 43 euros per share from 39 euros, representing a 35% premium to Qiagen's closing price when it made the initial offer on March 2.

The revised offer values Qiagen at 11.3 billion euros and is backed by Qiagen's board.

Thermo Fisher has been under pressure from some Qiagen shareholders to sweeten its bid, particularly after the company said last week it was experiencing strong demand for products related to coronavirus testing.

One of those investors, hedge fund Davidson Kempner, said on Thursday that it would reject the new offer because it still "falls short of fair value". It said a price of between 48 euros and 52 euros a share was more appropriate.

Thermo Fisher Chief Executive Marc N. Casper said that industry conditions had changed considerably in recent months.

"After careful consideration, we've decided to increase our offer for Qiagen to reflect the fair value of the business given the current environment," Casper said in a statement.

Last week, Qiagen beat expectations with a preliminary 18-19% rise in second-quarter sales and a 68% rise in earnings per share amid strong demand for products used in coronavirus testing.

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

(Reporting by Caroline Copley; additional reporting by Alexander Huebner; additional writing by Tom Sims; Editing by David Goodman and Carmel Crimmins)