Royal Mail workers will stage two 48-hour strikes around Black Friday and Cyber Monday in an escalating row over pay, jobs and conditions.
It announced strikes around Black Friday on November 25 and Cyber Monday on November 28.
The union has withdrawn strikes planned for November 12 and 14, saying it wants to take more “proportionate” action.
An offer made by Royal Mail on pay and conditions on Monday was described as a “surrender document” because the union said it involved attacks on terms and conditions as well as redundancies.
The union will put the offer to its members in a ballot, recommending rejection.
Members will also be asked to deliver a vote of no confidence in Royal Mail chief executive Simon Thompson.
CWU general secretary Dave Ward said industrial action will continue in the run up to Christmas unless the dispute is resolved.
A Royal Mail spokesman said: “On Monday October 31, Royal Mail proposed a new pay-for-change offer to the CWU worth 9% over two years, despite making a loss of £219 million in the first half of the year.
“The CWU have been in talks with us at Acas and claim they are open to change but they now need to show it.
“Instead, the CWU have announced four days of strike action which will damage our business further at our busiest time of year.
“The CWU is playing a dangerous game with its members’ jobs and the future of Royal Mail.
“We urge CWU to withdraw these strikes for the good of our customers and our people.
“We apologise to our customers for the inconvenience the CWU’s continued strike action will cause.
“We are doing all we can to minimise delays and keep people, businesses and the country connected.”
The union said its members will strike on Thursday November 24 and Friday November 25 and on Wednesday November 30 and Thursday December 1.
The CWU’s postal executive will meet on Thursday to discuss further action during the Christmas build-up.
Mr Ward said: “Posties are in the fight of their lives against the Uberisation of Royal Mail and the destruction of their conditions.
“But 115,000 of our members will not just accept this war on their livelihoods and their industry.
“They will never give up the fight to protect this industry and to protect their hard-won working conditions.
“(Royal Mail chief executive) Simon Thompson has to either accept that or walk away; until he does one or the other, serious disruption will continue.”
CWU acting deputy general secretary Andy Furey added: “Simon Thompson’s plan is evident – they want to destroy this company as we know it.
“They want outsourcing, casualisation, the decimation of working practices and pay.
“But so many of our members have given their entire working lives to building this company.
“They deserve a much better deal than what is on offer, and Simon Thompson is on another planet if he thinks we’ll stop fighting to achieve that.”