The union claims that the employment of temporary workers to carry out the work of those that are taking part in a permitted industrial dispute is illegal, but Royal Mail countered that its use of temporary workers is "entirely in line with all employment law".
A spokesperson at the mail operator 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 laws.
"In addition, we normally use agency staff throughout the year to help cover sick and holiday absence, fluctuations in volumes and higher volumes at Christmas."
Representatives from the CWU and Royal Mail will resume talks today (3 November) after the union announced more strikes would begin this Friday (6 November).
More than 120,000 workers are set to walk out for 24 hours from 3am on Friday and then again for 24 hours on Monday (9 November).
Elsewhere, Royal Mail has said that the backlog caused as a result of the three-day strikes last week has since halved from 50m to 25m.
CWU to take legal action against Royal Mail
The Communication Workers Union (CWU) has said it will seek a High Court injunction this Friday to stop Royal Mail from employing 30,000 temporary staff to clear the backlog of mail caused by earlier strikes.
