Royal Mail is facing a four-day walkout by 115,000 staff after CWU union members voted overwhelmingly for strike action over a pay dispute.
Twenty-four-hour strikes have been called for 26 and 31 August 2022 for shifts beginning at or after 04:00 hours and 8 and 9 September 2022 for shifts beginning at or after 04:00 hours.
Royal Mail said if the strikes go ahead its UK business will be materially loss-making in the financial year to end March 2023.
The company said the union rejected an offer worth up to 5.5%, with talks ongoing for more than three months.
“The negative commercial impact of any strike action will only make pay rises less affordable and could put jobs at risk,” it said.
The CWU wants any pay deal to reflect the current cost of living where inflation currently is running at 9.4% and predicted to go higher.
Dave Ward, CWU general secretary said: "Nobody takes the decision to strike lightly, but postal workers are being pushed to the brink.
"There can be no doubt that postal workers are completely united in their determination to secure the dignified, proper pay rise they deserve," he said.
Talking to the BBC, Ricky McAulay, operations director at Royal Mail said: "The CWU rejected our offer worth up to 5.5% for CWU grade colleagues, the biggest increase we have offered for many years," he said.
"In a business that is currently losing £1m a day, we can only fund this offer by agreeing the changes that will pay for it."
Royal Mail added it remains ready to talk with the CWU to try and avert the industrial action but any talks must be about both change and pay.
The postal service said its offer comprises an unconditional 2% pay increase, backdated to 1 April 2022 with a further 3.5% increase subject to agreement on working practice changes.
This current offer will cost Royal mail an extra £230mln, it added.