The union says it is ready to go to the High Court to stop the use of 30,000 agency workers because it breaches employment law.
Royal Mail announced last week it was taking on the extra staff, twice the number it would usually hire at this time of the year to cope with seasonal increases in mail, to avoid a backlog of mail and ensure services continue to operate.
The CWU hopes to demonstrate the move breaks employment law.
The Union’s legal plans come as it holds a second day of talks with Royal Mail bosses aimed at averting the strike action planned for later this week.
Up to 120,000 postal workers plan to strike on Thursday, Friday and Saturday this week over the dispute about pay, jobs and modernisation. The planned strikes follow the two-day strike last week.
The London Chamber of Commerce says last week’s walk-outs cost UK businesses an estimated £200m in lost income, while the DMA has estimated that the cost of lost, postponed or cancelled direct mail campaigns could total £10m.
The planned strike has prompted retailers including John Lewis, Amazon, eBay and Argos to discuss using alternative delivery services over fears that the strike will seriously disrupt business in the run-up to Christmas
However, online DVD rental firm Lovefilm, says it will continue to work with Royal Mail despite the disruption caused by the strikes, adding that the postal service has played a key part in its growth.