Strikes planned for the Black Friday weekend and the run-up to Christmas will go ahead after talks between Royal Mail and the Communication Workers Union (CWU) ended without agreement.
Deliveries will be disrupted by ten more strike days at the busiest time of the year for the postal service.
Royal Mail said it had made its "best and final offer" and accused the union of "holding Christmas to ransom".
But the CWU said the offer would "spell the end of Royal Mail as we know it".
Fresh strikes are scheduled for Thursday and Friday with further stoppages planned for 30 November and 1 December - just two days after Cyber Monday, one of the busiest online shopping days - and on six days in December, including Christmas Eve.
The long-running dispute revolves around pay, jobs and conditions. The CWU said Royal Mail's plans to change the way the postal system worked, would turn it into "a gig economy-style parcel courier, reliant on casual labour".
The company said in a statement that it had offered staff a 9% pay rise over 18 months, was committing to make Sunday working voluntary, and would make no compulsory redundancies before March next year.
"We want to reach a deal, but time is running out for the CWU to change their position and avoid further damaging strike action," Royal Mail's chief executive, Simon Thompson, said earlier.
Royal Mail said strikes had already cost the firm more than £100m and that the pay offer may need to be withdrawn if there is "further deterioration in the company's financial position caused by industrial action".
"In a materially loss-making company, with every additional day of strike action we are facing the difficult choice of whether we spend our money on pay and protecting jobs, or on the cost of strikes," Mr Thompson said.
The CWU represents 115,000 workers at Royal Mail. Last month they rejected a 7% pay offer over two years.
Zoe Conway, BBC Employment Correspondent
For both sides this dispute is existential. That's why it's so hard to solve. Royal Mail says it is in such financial peril that without immediate changes to working practices it could go out of business. For the Communication Workers Union, this is a last stand to stop postal workers being turned into "gig-economy couriers".
Letters don't make the company money. People are sending 60% fewer of them than they were in 2005. So the company wants to switch its focus to parcels and in particular to next-day parcel delivery. That means changing working practices.
Pay is of course an issue. The company says it's offering 9% more over 18 months with conditions attached, though the union disputes this figure.
Royal Mail also wants to end the extra payments that postal workers get for doing things like driving bigger trucks. Postal workers I've spoken to say they'd lose £50 a week if those allowances were to end. The company is offering to compensate them.
Both sides have incurred heavy losses. Royal Mail says eight days of strike action has cost it £100m whilst striking posties have each lost, on average, £1,000. The mood is very sour indeed.
The union criticised Royal Mail's "aggressive" stance over the talks and called for an improved pay deal, a guarantee of no compulsory redundancies and other improvements to the offer.
CWU general secretary Dave Ward said the offer represented a "devastating blow" to postal workers' livelihoods and urged the public to "stand with their postie".
"These proposals spell the end of Royal Mail as we know it, and its degradation from a national institution into an unreliable, Uber-style gig economy company," he said.