Royal Mail workers are to hold 19 days of strike action over pay and conditions during the peak postal build-up to Christmas.
The Communications Workers Union CWU said the strikes will cover Black Friday and Cyber Monday.
It said it reflects the level of anger workers feel.
Royal Mail Group said further strikes would make its financial position worse and threaten job security.
Strike action will take place from 04: 00 on various days starting on October 13th, beginning Black Friday week and Cyber Monday.
All workers will have to take action on October 13, 20, 25, and October 25 and November 28.
The teams and combinations of teams will strike on other dates but that will have a knock-on effect on all deliveries on those dates, the CWU spokesman said.
Dave Ward, CWU's general secretary, said: This is a significant announcement, but it is one that matches the level of anger our members feel at the way Royal Mail Group has treated them. These are the same people who have kept the country connected and returned the Royal Mail Group to record profits, he said.
Postal workers in the UK are facing the battle to save their jobs and provide the service they provide to every household and business in the UK.
We want everyone to stand with their local postal worker. A spokesman for the CWU said Royal Mail's management was trying to casualise the postal service.
He said they want to turn it into an Uber-style employer.
We operate in a competitive market, and our customers have choices.
Our financial position will be worse because of further strikes and resistance to change by CWU, and threatens the long-term job security of our postmen and women. The spokeswoman said Royal Mail had invited CWU to discussions through the dispute resolution service Acas on 22 September.
Rather than responding to our offer of Acas talks, the CWU announced further damaging industrial action, once again taking the path of prolonging disruption over resolution. The spokesperson said that they were doing everything possible to minimize any delays and keep people, businesses and the country connected. Julia Kermode, founder of IWORK, said on the strike announcement: What did the government expect? In the mini-Budget, the Chancellor promised to make it harder for workers to strike. Royal Mail workers have announced they will strike at the busiest time of the year, amid soaring inflation and huge economic uncertainty created by his potentially disastrous fiscal package.
It means that Royal Mail will lean more heavily on the support of temp workers. The postal service will benefit from new legislation that allows agency temp workers to cover for striking staff. The new legislation was rushed in recently to minimize the impact of strikes on middle England but unions are challenging its legitimacy, which they say is contravenes to international labour law.
If Royal Mail decides to use temps to cover their striking employees, it will need to be sensitive. Innocent workers should not be sent into a potentially volatile situation. The temps are aware of what they are walking into, so care needs to be taken to make sure they are aware of what they are walking into. Some may prefer to work elsewhere than to cover for strikes and it is important that they are given the opportunity to make an informed decision.
It is important that these workers are paid and treated fairly. The moment when temps and agency workers are given a raw deal is when vital sectors grind to a halt.