Postal workers will individually take two more days of strike action in the next two weeks, but the new action means there will be two weeks of continuous disruption to mail services across the country.
The strikes call out separate functions nationally on different days, with the timing aimed at maximising the impact on mail deliveries.
Dave Ward, CWU deputy general secretary, said: “We have contacted Royal Mail leaders on [numerous] occasions, consistently asking for negotiations to find a resolution to the dispute.
“Royal Mail management continues to ignore the union and its workforce. This is absolutely unacceptable.”
In addition to strike action, the CWU is stepping up its political campaign to increase public awareness of the issues behind the dispute.
The union has already held two one-day strikes over the past month.