Royal Mail workers will stage two 48-hour strikes around Black Friday and Cyber Monday in a row over pay, jobs and conditions.
The Communication Workers' Union (CWU) will recommend around 115,000 of its members reject a pay offer of around 9% spread over two years.
But the union cancelled two strikes on 12 and 14 November, saying it wants to take more "proportionate" action.
Royal Mail urged workers not to strike at the "busiest time of the year".
The union called Royal Mail's pay offer "derisory", and said the strike days include two of the the most lucrative weekends of the year for online shopping and promotional discounts at retailers.
Workers will be walk out on 24 and 25 November - known as Black Friday - and then again on 30 November and 1 December - just two days after Cyber Monday, one of the busiest online shopping days.
A Royal Mail spokesperson said: "Royal Mail proposed a new pay-for-change offer to the CWU worth 9% over two years, despite making a loss of £219m in the first half of the year.
"The CWU is playing a dangerous game with its members' jobs and the future of Royal Mail. 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", they added.
The CWU's general secretary Dave Ward said the 48-hour strikes were partly in protest at the "Uberisation" of the postal service, including "widespread changes... introducing Uber-style owner-drivers, mail centre closures and changes to Sunday working".
Union members will also be asked on Thursday to deliver a vote of no confidence in Royal Mail chief executive Simon Thompson.
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