Royal Mail workers are to go on strike on 19 days over the next two months in a dispute over pay and conditions.
The dates will be spread across October and November, the Communication Workers Union (CWU) said, and they will be a mix of single days and rolling action across Royal Mail Group's network.
It means the fresh walkouts by postal workers will cover peak delivery periods like Black Friday and Cyber Monday, as well as some of the run-up towards Christmas, and the CWU said it will have a "dramatic impact".
The CWU said the move follows a threat centred around the "outrageous" decision by Royal Mail Group's senior management to withdraw from major national agreements, push ahead with cuts to workers' terms and conditions, and "completely sideline" the union.
Some 115,000 CWU members were already preparing to walk out for 48 hours on Friday 30 September and Saturday 1 October after that action was revealed earlier this month.
There have already been a number of other strikes by Royal Mail workers this year.
'Workers face the fight of their lives'
CWU general secretary Dave Ward: "This is a significant announcement, but it is one which matches the level of anger our members feel at the way Royal Mail Group has treated them.
"The CEO of Royal Mail Group, Simon Thompson, is treating postal workers as if they are stupid.
"These are the same people that have kept the country connected and returned Royal Mail Group to record profit.
"Postal workers across the UK now face the fight of their lives to save their jobs and the service they provide to every household and business in the UK."
A Royal Mail spokesperson said: "On 22 September, Royal Mail invited CWU to enter into talks through Acas (the arbitration service) to find a resolution to our dispute on change and pay.
"This evening, 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."
Royal Mail 'losing £1m a day'
The spokesperson also said Royal Mail is losing £1m a day and "must change faster in response to changing customer demands".
"Further strikes and resistance to transformation by CWU will only make our financial position worse, and threatens the long-term job security of our postmen and women," they said.
The spokesperson apologised for the "inconvenience" that the strike action will cause and said the company was doing all it could to minimise any delays.