We started to get a barrage of calls over the past month, which has consistently grown, said South of England branch senior organiser Jamie Major.

Two and a half years ago 262 workers at the plant signed a petition demanding better working conditions. Since then the GPMU has been recognised and conditions had improved, but Major said the company now seemed to be slipping back into its old ways.

He said the latest complaints included a female Albanian employee allegedly being threatened with dismissal for not speaking with an English accent.

Despite calls to Gardners Books, no one was available for comment.