The Royal Mail is to face legal action over the recruitment of 30,000 agency staff during the post strike, union leaders announced tonight.
The Communication Workers Union served notice of action in the High Court on Friday after considering the move for the past few weeks.
The company has denied the 30,000 workers are being used to break the strikes, insisting they are dealing with the backlog of post as well as preparing for the Christmas rush.
Meanwhile talks between the two sides to try to break the deadlocked row over jobs, pay and modernisation are to resume today.
The Government was earlier threatened with legal action if it did not investigate claims that agency staff have been unlawfully hired to do the work of striking postal workers.
The GMB union wrote to Business Secretary Lord Mandelson calling on him to investigate the role of employment agencies which have supplied temporary staff to the Royal Mail.
The union believes the law has been broken in recent weeks because it is illegal to take on staff to do the work of people involved in a lawful industrial dispute.
The GMB demanded an investigation into alleged breaches of the law in Slough, Bristol and Dartford, warning it did not rule out taking enforcement proceedings against Business Department and the Government if they failed to act.
General secretary Paul Kenny said in a letter to Lord Mandelson that there were specific cases involving agency staff being taken on in recent weeks seemingly to do the jobs of strikers, which needed to be investigated as a matter or urgency.
Mr Kenny said in his letter: "There is an agency in the department you run that has a duty to investigate breaches and it has a very poor track record in enforcing the law.
"I am calling on you and your department to instruct the agency to investigate these and other cases and to properly enforce the law for which you and your department are responsible.
"GMB reserves the right, if you and your department fail to carry out your lawful responsibilities or act in a prejudicial way, to seek enforcement proceedings against your department and the Government."
The move came as both sides in the postal dispute continued to study proposals drawn up by TUC general secretary Brendan Barber aimed at resolving the dispute.
The Royal Mail said the backlog of post caused by three days of strike action last week had halved to 25 million today, but further walkouts are planned for Friday and next Monday unless the deadlock is broken.Reuse content