The union has accused the postal operator of aiding and abetting what might be a criminal activity as it seeks to deal with seasonal lifts in volume and the backlog of mail created by striking workers by employing agency workers.
 
The CWU claims that, in doing so  the agency  is breaching Regulation 7 of the Conduct of Employment Agencies and Employment Businesses Regulation 2003  which prevents  employment agencies  from  supplying staff to cover the work of striking workers.
 
Breaching this rule is a criminal offence, and the CWU claims that if this is the case in this instance, Royal Mail is a party to it. It is seeking an injunction to prevent this continuing at the High Court.
 
However, the Royal Mail has an arrangement with the employment agencies it is using that ensures that, while the agency staff are sourced by the agency, they are contractually employed by Royal Mail.  This may not be a breach of the Regulations and it is not illegal for companies to directly employ staff to cover work during strike action.  The Royal Mail also say that the workers are not doing the work of the strikers per se.
 
Even so, in its action, CWU infers that the principle of Regulation 7 is to prevent companies quickly engaging large numbers of workers to take over the roles of striking staff, as employers would naturally struggle to achieve this by sourcing them directly. It claims that Regulation 7 should be read more broadly to interpret Royal Mail's strategy as breaking this rule, although the Regulation has  not been tested in this way before.

"This is ground-breaking stuff, definitely," said Joan Cradden, head of employment unit Brodies LLP. "The arguments on both sides are  interesting and ultimately political.
 
"Sometimes it's not clear who employs the agency workers and their contractual status can be pretty nebulous. That has undoubtedly fuelled the debate on the use of Regulation 7."
 
The union has chosen to pursue an injunction against Royal Mail rather than lobbying the Employment Agency Standards Inspectorate to investigate the agencies concerned and recommend criminal proceedings against them. The CWU and other unions have been critical of the Inspectorate's failure to date to refer any breach of Regulation 7 to the public prosecutor.
 
The Employment Agency Standards Inspectorate is part of the Department of Business, Innovation and Skills, currently headed by Lord Mandelson.