Solidium became an investor in Nokia in 2018 after buying a 3.3% stake for about 844 million euros (753.48 million pounds), and has been increasing its stake since late last year.

Nokia, which axed its dividend after a profit warning last October, has been scripting a turnaround by keeping a lid on costs and addressing shipment delays in its attempt to compete with rivals such as China's Huawei and Sweden's Ericsson.

Solidium had criticised Nokia earlier this year for poor communication and sent some "feisty feedback" to the company's management following its profit warning.

The Finnish telecoms equipment maker has since named Sari Baldauf as chair and appointed Pekka Lundmark as its chief executive.

Nokia shares on Thursday closed at 39.42 SEK.

(Reporting by Supantha Mukherjee in Stockholm; Editing by Leslie Adler)