Contract workers have already staged one unofficial strike at Aberthaw

The workers, employed by a contractor at the Aberthaw Power Station, complained about different pay rates to employees of another sub-contractor.

Engineering staff, employed by a manufacturing support subcontracor, picketed the plant on Monday.

It followed an earlier stoppage on Friday.

The current dispute is not with RWE npower, the owners of the station, but with some of the contractors undertaking maintenance work.

The strike action is unofficial and it has not been supported by the two trade unions involved, Amicus and the GMB.

Both unions may now make arrangements for an official ballot.

The workers claim the sub-contracting company has refused to implement conditions at Aberthaw which were agreed nationally.

No-one from that firm was available for comment.

GMB union representative Mike Payne said: "Unofficial action was taken by 200 Amicus members on Friday, which stopped GMB members going into work.

"[The Amicus official] said he would be going back on Monday to ask them to go back to work," he said.

A spokesman for npower, which operates the power station, said it was encouraging the two sides to resolve the dispute as soon as possible.