About 42,000 mail centre staff and drivers will walk out next Thursday (October 22) for 24 hours, followed by another 24-hour stoppage on Friday (23) by 78,000 delivery and collection staff.
The Communication Workers Union said it had made a "genuine offer" to avert a national strike and accused the Royal Mail of rejecting it without proper consideration.
The union said it had "no choice" but to press ahead with naming strike dates after its peace offer was turned down by the Royal Mail.
Union members voted by 3-1 to take national industrial action following a spate of regional strikes in the long-running row.
Dave Ward, CWU deputy general secretary, said: "We made a genuine offer to Royal Mail that would have given space for detailed discussions without a strike.
"We were severely disappointed that within two or three hours the company rejected it, apparently without even affording it proper consideration. This comes on the back of several rejections of CWU offers and is a clear indication that they are not trying to reach agreement.
"Our offer included the opportunity for a three-year deal that would bring stability to customers, business and the workforce. This industry is crying out for stability and yet Royal Mail rejects out of hand a genuine attempt to deliver it.
"We reiterate that offer today and ask that Royal Mail look again. Making progress on some key elements of that offer would mean an immediate strike could be avoided."
The union said it was still prepared to explore third-party mediation, and revealed that talks with the Royal Mail had been held today.
The CWU also called on the Government to intervene in the dispute, warning that the Royal Mail's pension deficit was now expected to exceed £10 billion.
Mr Ward said: "This Government has recently intervened in the problems of Vauxhall and Lloyds Bank. They are the sole shareholder of Royal Mail and it is unbelievable that they continue to stand aside.
"If Royal Mail is not prepared to reconsider our offer and negotiate around that offer then we are strong, resolute and the strike will go ahead."
A total of 83 MPs have signed a Commons Early Day Motion calling on the Government and Royal Mail to support a peace deal.
Royal Mail's managing director, Mark Higson, said the CWU's decision to call a national strike next week was an "appalling and unjustified attack" on customers and showed a "reckless disregard" for everyone who depended on the company.
"Customers large and small have been hoping the CWU would lift the strike threats and focus on providing the service they need and want - instead the union has given them a slap in the face.
"The CWU's strike announcement simply shows just how dishonest the union's claim to embrace the need to modernise is, and underlines the union's opposition to simple changes such as its members working flexibly for all of the hours they are paid and using the equipment provided to do the job.
"Instead the CWU is demanding an absolute veto over future change and modernisation - and demanding more money - backed up with strike action which they know will drive customers away."
The Royal Mail accused the union of reneging on its commitments, repeating that the firm would be making no further changes to its operations this year.
"Since then the union has not only failed to deliver on a moratorium, but has called strikes in more than 150 delivery offices and has now announced a national strike which will further damage our customers and the entire postal service.
"The union seem to be in denial about the reality of the tough economic conditions facing everyone in the UK and the impact of competition, especially from email and the internet, which has helped drive mail volumes down by around 10% this year."
Business Secretary Lord Mandelson said: "I very much regret this decision by the CWU. Candidly, I think it is suicidal.
"Taking industrial action will not resolve this dispute. It will only serve to drive more customers away from Royal Mail. One thing this company cannot afford is strikes and industrial action.
"We are, of course, in frequent contact with both management and the union. Our message to them has been clear: put your customers first.
"The CWU should turn their backs on industrial action and sit down with the Royal Mail and resolve this dispute. That is what is in the interests of the Royal Mail, their members and the country."
Vanessa Canzini, spokesman for online auction site eBay, said: "In the four months since the regional strikes began and in the seven days since the Communication Workers Union announced their intention to strike, there has been no noticeable impact on the number of items being bought and sold on eBay.co.uk. We believe this demonstrates that confidence in shopping online remains strong.
"That said, ensuring our customers - buyers and sellers - continue to enjoy the best shopping experiences is paramount.
"Although many of the items for sale on our site already offer alternative postal carrier options, we've negotiated alternative providers and special rates on behalf of our sellers, so they can continue providing the outstanding service our buyers currently enjoy.
"These special rates are open to any seller, meaning that even hobby sellers wishing to sell on the site in the run-up to Christmas will be able to secure a door-to-door courier service for just a few pounds."
Robert Hammond, of campaign group Consumer Focus, said: "This is the news we'd all hoped to avoid. Efforts to seek urgent, independent mediation should be redoubled.
"Neither the protagonists nor Royal Mail's customers have anything to gain from this strike.
"Both parties must do all they can to avoid a protracted dispute if we're to avoid a disastrous Christmas for consumers and businesses alike."
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