Nearly 10 percent, or 150, of workers at the Stora Enso paper mill in Oulu have tested positive for coronavirus, the company announced on Wednesday.
The firm said that all of its more than 1,700 workers at the facility have been tested and that some of those who tested positive were asymptomatic.
Last week, the forest products company shut down some paperboard production following a coronavirus outbreak at its Oulu mill.
"We don't know where the infection came from but it seems to mostly have spread in an area [of the facility] where around 600 people work," the mill's chief Juha Mäkimattila said in a statement.
The first coronavirus infections detected among staff members were diagnosed at the end of last month but the number of cases have decreased in recent days, according to the firm.
The facility is currently in the process of being converted into a packaging board mill, an effort involving around 1,800 outsourced workers. The work is being carried out at two separate sites.
The company said it plans a second round of Covid tests of workers at the end of the week.