More than 115,000 members of the Communication Workers Union staged a sixth day of strikes this week. The union has announced 19 further days of action in the coming weeks.
It comes amid a dispute with Royal Mail over pay, conditions and working arrangements.
With a stand-off between the two sides, Royal Mail said it was taking action “to better match our costs to current parcel and letter volumes.”
It confirmed plans to cut its 140,000 workforce by 10,000 by August next year.
Some of those job losses are expected through measures such as workers leaving of their own accord.
But the company warned that would still mean between 5,000 and 6,000 redundancies may be needed.
Royal Mail added that its voluntary redundancy offer of up to two years’ pay was “now unaffordable” and would be ditched.
Chief executive Simon Thompson said: “It totally saddens me that our financial position and the lack of progress on transformation means we are in this position where we urgently have to right-size our business.”
Mr Thompson said the 10,000 job cuts were based on the strikes that had already happened and the financial hit to the firm.
He added: “If there is more industrial action there will be more jobs at risk.”
It came as the company said it lost £219million in the past six months, which could reach £350million over the full year.
“This may increase to around a £450million loss if customers move volume away for longer periods following the initial disruption,” it said.
Royal Mail recently ditched its more than 500-year-old name and replaced it with International Distributions Services for its parent company.
Bosses have already warned the group, which also includes international parcels arm GLS, could be split entirely.
International Distributions Services said GLS would not be used to “cross subsidise” the Royal Mail arm.
CWU General Secretary Dave Ward said: “The announcement is the result of gross mismanagement and a failed business agenda of ending daily deliveries, a wholesale levelling-down of the terms, pay and conditions of postal workers, and turning Royal Mail into a gig economy style parcel courier.
“What the company should be doing is abandoning its asset-stripping strategy and building the future based on utilising the competitive edge it already has in its deliveries to 32 million addresses across the country.
“The CWU is calling for an urgent meeting with the Board and will put forward an alternative business plan at that meeting.
“This announcement is holding postal workers to ransom for taking legal industrial action against a business approach that is not in the interests of workers, customers or the future of Royal Mail. This is no way to build a company.”