The Finnish Paper Union members will walk out on six of UPM's Finnish pulp and paper mills in a 24-hour strike.
This excludes the company's Kajaani mill and its Tervasaari pulp mill, which UPM has said it plans to close later this year.
A UPM spokesman said it can take up to eight hours to shut down a paper machine and up to 12 hours to start back up.
It is not currently known how much tonnage will be lost following the strike, which is said to end at 6.00 tomorrow morning.
Last week, UPM announced the potential closure of its newsprint and graphic paper mill in Kajaani. This will take out 13% of its newsprint capacity in Europe, 25% of its special newsprint capacity, and around 9% of its SC (super-calendered) paper capacity.
The potential closure of its Tervasaari pulp mill will reduce UPM's pulp production by 10%.
European papermakers are facing steep energy and raw material costs and demand for newsprint in Europe has been reducing.
Other paper manufacturers, such as Stora Enso and M-real, have also announced large capacity cuts and staff lay-offs over the past year.
4,000 UPM staff strike following mill closures announcement
More than 4,000 UPM workers are on strike in Finland today after the paper giant announced mass restructuring plans that will see 1,600 staff laid off and two mills closed.
