More than 98% of those who voted backed taking strike action, the union said, in what would be the second walk out by more than 115,000 postal workers this summer.
"The ongoing attempts of Royal Mail Group to whittle away people's hard-won working conditions will be met with fierce opposition," CWU Deputy General Secretary Terry Pullinger said in a statement.
The strikes are the latest in a wave of industrial unrest in Britain that has affected a range of industries as wages fail to catch up with soaring inflation that topped 10% in July.
More than 115,000 Royal Mail workers are already due to strike in a separate dispute over pay on Aug. 26 and 31, followed by two more days early next month, in possibly Britain's biggest strike this summer.
Royal Mail, one of the world's oldest postal groups, has said it had offered a 5.5% pay rise for CWU-grade workers, its biggest increase in years.
(Reporting by Sachin Ravikumar; Editing by Kylie MacLellan)
