The two proposed 48-hour strikes, which will take place on October 5-6 and on October 8-9, have been condemned by Royal Mail, which said that CWU "does not begin to understand the challenges facing Royal Mail" and warned of "serious consequences" if the business failed to modernise.
CWU has accused Royal Mail of adopting "draconian and destructive" proposals, and said it is an employer "completely out of control" for not improving April's 2.5% pay rise offer and what it said was a decreased pension benefits scheme.
The strikes are the latest wave of industrial action to be taken by CWU in response to a fall-out with Royal Mail over working conditions, modernisation and pay, which began in June.
In addition, CWU said it planned a rolling programme of "functional strikes" which would begin on October 15 until the present dispute was resolved.
In a statement, Royal Mail said: "The union's repeated refusal to contemplate the changes we must make if we are to be able to compete in an increasingly tough market simply shows that [CWU] are completely out of touch with UK industry and the reality of how competitive markets work."
David Ward, deputy general secretary at CWU, countered: "Despite five weeks of negotiations, Royal Mail has failed to take on board the union's message that in order for the business to succeed Royal Mail need to invest in their workforce.
"Strikes are a proportionate response to an employer this is completely out of control. Rather than running the business, Royal Mail's actions demonstrate they are intent on destroying it."
Royal Mail has said CWU has misled union members over its pension proposals by claiming executive action over a future deal had already been taken. Royal Mail said the official 60-day consultation period would start at the beginning of October, and would take into account all views of employees and stakeholders.
Despite each party making clear its stance over pay and modernisation, Royal Mail and CWU do not appear to be nearer reaching an agreement to end the strikes.
Ward said: "The workforce has had enough of Royal Mail messing them around. We're not going to tolerate an employer that ignores its workforce, ignores the union and ignores its customers."