Last ditch talks aimed at preventing crippling post strikes are to be held as union leaders consider a legal challenge to the recruitment of up to 30,000 temporary staff by Royal Mail.

The company, facing two national walkouts on Thursday and Friday, announced it was hiring twice the number of extra staff it usually takes on in the run-up to Christmas.

The leader of the Communication Workers Union said the move would “inflame” the long running dispute over jobs, pay and services and repeated his call for outside mediators to be brought in to break the deadlock.

CWU officials said they believed the recruitment of temporary staff during a strike was illegal and they received backing from other union leaders.

“We will be looking at the legal side of this and we are calling on recruitment agencies to be aware of the law if they are asked to supply temporary workers to the Royal Mail,” said one CWU official.

Regulations amended in 2007 state that 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.

CWU general secretary Billy Hayes said the recruitment announcement was typical of the culture of management at Royal Mail. “I think it is a stupid move. It will inflame things,” he said on BBC TV’s Andrew Marr programme.

Royal Mail said that as well as dealing with the extra mail sent in the weeks running up to Christmas, it wanted to offset the impact on customers of the CWU’s “unjustified and irresponsible” strikes.

The recruitment drive was not aimed at bringing in staff to do postmen’s work when they are out on strike, but to make sure there were enough employees to help clear any backlogs between walkouts as well as to help with the seasonal build-up of mail, said the company. Managers said the recruitment was fully in line with all employment law.