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 on November 25 and 26, affecting Black Friday and on November 30 and December 1, timed to disrupt Cyber Monday.
The union has withdrawn strikes planned for November 12 and 14, saying it wants to take more "proportionate" action.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.
Dave Ward, the CWU general secretary, 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 per cent 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."
