The Communication Workers Union (CWU) will go to the High Court this Friday to seek an injunction to stop Royal Mail using 30,000 agency workers to clear backlogs of post.
The union has said the postal service is using the agency workers as strike breakers as they have been made to carry out the work of striking Royal Mail employees.
Royal Mail has denied these allegations and said its use of agency workers had been "entirely in line with all employment law".
A spokewoman for Royal Mail 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 the agency workers were not doing the work of postal workers on strike.
Guy Lamb, an employment partner at legal firm DLA Piper, told Personnel Today the CWU's legal challenge was "unlikely" to succeed.
He said: "Despite the rhetoric of the CWU, 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."
Lamb added that the agencies, including Jobcentre Plus, were regulated by the Department of Business so any legal action against them would be the responsibility of the business secretary Peter Mandelson "who has already stated that he doesn't believe any regulations have been broken".
He added: "It is unlikely that the Minister for Business will want to embroil himself in this issue unless absolutely necessary, considering the impact of the strikes on the UK business community and economy."
Royal Mail workers are expected to stage an all-out strike strake on Friday and on Monday, 9 November, with up to 121,000 union members walking out on each of the days.
The dispute is over pay and modernisation at the Royal Mail.