The Paperworkers' Union announced on Thursday that UPM plant workers will extend an ongoing strike until the end of April, unless a deal with the company over a new collective labour agreement can be reached.
The strike was due to end by mid-April, but negotiations between the forest products firm and the union over pay and working conditions within the industry have failed to yield results.
The strike began on 1 January and involves about 2,000 workers across 12 paper mills.
From the outset of the talks UPM have said they would prefer to negotiate unit-by-unit, as the company argues the businesses differ from each other so vastly that a single collective agreement would not be applicable.
Having initially resisted this approach, the union agreed in February to negotiate on a unit-by-unit basis.
However, the Industrial Union stepped in to fund strike pay for Paperworkers' Union members, a signal that the dispute was more entrenched than ever.