GPMU national industrial officer Peter Ellis said members were being balloted on taking industrial action.

"There seems to have been a number of problems which came to light after the last dispute," said Ellis.

These are believed to concern the non-payment of additional payments made to workers for working different shifts. Ellis said workers were currently forgoing these additional payments.

GPMU deputy general secretary Tony Burke said: "Since the company conceded union recognition the GPMU has been working hard to try to reach an agreement on proper working conditions within the factory.

Ellis said he was hopeful that industrial action could be avoided.

Talks were still available and he said ACAS is also set to become involved.

Despite calls to management at the firm's Yardley headquarters, no one was available for comment.

Workers at the Sparkbrook-based plant took a series of 48-hour and 72-hour stoppages following the breakdown of talks in April this year.

Story by Andy Scott and Josh Brooks