Some 1,500 staff across the two companies went on strike this morning, after a planned two-week walk-out was moved forward one day by the Union in order to pressure government mediated bargaining.

All Stora Enso and Metsäliitto workplaces including Metsa-Botnia, M-real and Metsa Tissue are affected by the strike, which started today at 6am and will go on for two weeks and a day.

It follows the end of UPM's salaried employees' two-week strike last week, which involved 1,000 workers and resulted in a EUR1m (£887,000) hit for the company.

Dick Blin, pulp and paper officer at ICEM, said that there should be no surprise that Stora Enso has been targeted for the next wave of full strikes, after the company allegedly attempted to force overtime on employees during a lawful overtime ban at the beginning of April.

He said: "The discontent by union members against Stora Enso's heavy handed tactics in trying to break a lawful overtime ban is expressed in this call for an extended strike."

He alleged that last week, 35 women walked out at Stora Enso's global financial centre in Kotka after being ordered to work longer hours to train Estonian workers brought in in preparation for the current two week strike.

Päivi Kauhanen, director of communications in Finland, said: "We have 25 people, not 35 women, going into illegal strike in Kotka.

"They walked out because we are planning to train Estonian people in Estonia to serve as back-up for the Kotka centre in the future, and it is nothing to do with the strike."

She said the paper manufacturer doesn't yet have the exact figures as to how many people are on strike.

"There is no direct effect at the moment because of the strike. We are trying to make sure that we can continue to serve our customers," she added.

A spokeswoman for Metsäliitto said that as the strike has just started, she was unable to confirm how many people were taking part at this time.