Twenty-one staff at Burnley Conversion, formerly the Papermarc Group, all of whom are Amicus members, returned to work to find the doors locked at the plant.
Around 90 staff at Papermarc Merton in Aberbeeg, South Wales have also been locked out of their site, which makes cartons and cases for the food industry.
The Papermarc group went into administration in August last year, with 300 jobs lost and BDO Stoy Hayward appointed as administrators.
A fire in March was blamed for the company's difficulties.
A local management team took over the running of the Burnley operation in December last year, after the mill was closed.
Amicus officer Louisa Bull said that despite repeated requests for information on the owners of the new company and the plant's future, none had been forthcoming.
"Our members have been treated with utter contempt by whoever is behind this," said Bull. "They have endured months of insecurity through and this is how they are being treated."
No one from Burnley Conversion was available for comment.
Story by Andy Scott
