Royal Mail has asked permission from the Government to stop delivering letters on Saturdays.
The proposed five-day letter service will take place Monday to Friday after it was revealed that the group suffered a huge £219 million loss in profits - with industrial action set to affect this even more.
There are still several strike days planned before the year is over. It's estimated that the five strikes in October alone cost the company about another £30 million.
Royal Mail is aiming to keep its seven-day parcel service, but it has formally requested that the letter service be slashed to just five days under its Universal Service agreement.
The CWU has been involved in a long-running dispute with Royal Mail this year over working conditions, job losses and pay.
Further strikes are expected around Black Friday and Christmas, with people warned of severe disruption to their deliveries.
Simon Thompson, chief executive of Royal Mail, said: "We have always been clear we need change to survive. We have started turning the business around and will do whatever it takes."
He added: "We would prefer to reach agreement with the Communication Workers Union (CWU) but in any case we are moving ahead with changes to transform our business.”
Royal Mail's owner, International Distributions Services (IDS) - which was renamed from Royal Mail Group earlier this year - includes international delivery service GLS and Intragroup which reported pre-tax losses of £127 million for the first half compared to profits of £315 million a year ago.
Keith Williams, non-executive chairman of IDS, said: "We have started to implement the change needed to rightsize Royal Mail, which will ensure that it is both better placed to serve our customers’ needs in parcels, as well as letters, bring it back to profitability and provide a sustainable future."
It comes after Royal Mail recently warned it may be axing thousands of jobs to combat the company's loss in profits.
Last month the postal firm said it would consult on up to 6,000 redundancies in efforts to reduce 10,000 full-time roles by August 2023.
It has so far focused efforts on the removal of overtime, the decision not to fill empty roles and a reduction in temporary workers, but gave no update on how many roles have gone.
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