The stevedores' strike, which began today (4 March) at 6am local time, comes after talks between port operators and union officials collapsed last night.
UPM has been forced to close its Rauma paper mill, where all four magazine paper machines are now shut down. Preparation for machine line shut-downs at the company's other mills is ongoing with the last paper machine closures expected to take place at the beginning of next week. UPM said it would try to operate its Finnish pulp mills until the end of next week.
The financial impact of the shut-down on the company's results is predicted to exceed €3m (£2.7m) per day. Some 3,000 staff will also be affected.
UPM's senior vice-president for paper manufacturing, Arto Lampinen, said: "In recent years, our paper mills have worked hard to improve their competitiveness. This strike destroys the good results of that work."
Stora Enso has also begun the temporary shut-down of machines at its Finnish mills. The impact on the company's financial results is estimated to be €2.5m per day.
Juha Vanhainen, Stora Enso country manager for Finland, said: "The impact of the strike will be wide and indefinite.
"It will severely tarnish the reputation of the Finnish forest products industry and its whole value chain."
The strike follows a one-day stoppage by Finnish road transport workers that halted transportation of raw materials to mills.
Today's strike is another blow to the beleaguered Finnish paper industry, which has been hit by overcapacity and raw material price increases over the past few years.
Yesterday, UPM's general manager, Will Oldham, said he did not know how long the strike was likely to last, but that as the impact on the Finnish economy would be "very significant, every political measure will be taken to ensure a quick resolution".
"In the meantime, we will use both UK stocks and UPM mills outside Finland to minimise disruption to customers. We are talking with every potentially affected customer to assess the best options for their particular requirements."
