The Communication Workers Union will go the High Court on Friday seeking an injunction against Royal Mail's use of agency staff to undermine the postal strike.

The union argues that the company's use of 30,000 agency workers to clear the backlog of mail caused by the strike is unlawful.

Royal Mail said its use of temporary staff was "entirely in line with all employment law".

CWU members are due to hold two further rounds of strikes on Friday and next Monday in the hope of reaching agreement before Christmas.

A Royal Mail spokeswoman said: "The up to 30,000 directly engaged, fully vetted temporary workers being hired by Royal Mail to help us clear any mail delayed by strikes, and help with Christmas volumes are entirely in line with all employment law".

She added: "In addition we normally use agency staff throughout the year to help cover sick and holiday absence and fluctuations in volumes, as well as higher volumes at Christmas.

"These agency workers are not hired to do the work of postal workers when they are on strike and also completely in line with employment law."

But the union accuses Royal Mail of using agency workers as strike breakers and argues that under regulation 7 of the Conduct of Employment

Agencies and Employment Businesses Regulations 2003, "an employment business may not supply a temporary worker to a hirer to replace an individual taking part in an official strike or any other official industrial dispute".

But Guy Lamb, employment partner at DLA piper, said that Royal Mail had not broken employment law.

He said: "Despite the rhetoric of the Communication Workers Union, Royal Mail is not directly breaking any law by bringing in temporary workers during strike action. The laws to prevent the use of temporary workers to cover striking staff actually only apply to the employment agencies who provide the workers."

Talks between the two sides over job losses and pay levels will resume today.

Royal Mail said the number of letters delayed as a result of the latest strikes had fallen from 50m at the weekend to 25m.