LONDON (AFP) - The postal union announced late Monday legal action against state-owned postal service Royal Mail as a dragging strike over pay, working conditions and modernisation took a new twist.

The Communication Workers' Union (CWU) said it was taking action in the High Court after Royal Mail hired 30,000 temporary workers to try to reduce a backlog of millions of letters caused by the long-running dispute.

The GMB, Britain's general trade union, earlier threatened Business Secretary Peter Mandelson with action if his department did not investigate claims the workers had been hired as strike breakers.

"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," GMB general secretary Paul Kenny said in a letter to Mandelson.

Royal Mail said a backlog of post caused by three days of strike action last week had halved from 50 million to 25 million on Monday. But further walkouts are planned for Friday and next Monday unless the deadlock is broken.

Both sides have blamed the other for the strike action, which comes as Royal Mail is losing business as fewer people send letters and competition from the private sector grows.

A three-day strike was called by the CWU last week after talks with Royal Mail broke down, just days after a 48-hour stoppage the week before caused widespread disruption.

Royal Mail has denied that 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.

The legal action comes as both sides in the dispute continue to study proposals drawn up by union leaders aimed at resolving the dispute.