Talks to try to break the deadlocked postal dispute are expected to resume as the Royal Mail faces legal action over the recruitment of 30,000 agency staff during the strike.
The Communication Workers Union (CWU) served notice of action in the High Court on Friday after considering the move for the past few weeks.
The company has denied that the 30,000 workers are being used to break the series of post strikes, insisting they are dealing with the backlog of post as well as preparing for the Christmas rush.
Meanwhile, talks between the two sides aimed at resolving the row over jobs, pay and modernisation are to resume on Tuesday, sources told the Press Association.
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 the Business Department and 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 of urgency.
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.