Not all union members will be striking at the same time
The Communication Workers Union (CWU) has said two 24-hour nationwide postal strikes will start on Thursday 22 October.
The union said it had no choice but to announce a strike after the Royal Mail rejected its latest set of proposals.
On the first day, mail centre staff and drivers will strike. The next day it will be delivery and collection staff.
Royal Mail condemned the strikes as "an appalling and unjustified attack on customers".
"Customers large and small have been hoping the CWU would lift the strike threats and focus on providing the service they need and want," said Royal Mail managing director Mark Higson.
"Instead the union has given them a slap in the face."
Business Secretary Lord Mandelson told the BBC the union's decision to strike "amounts to a death wish".
The CWU said talks were continuing and strike action could still be avoided.
"We made a genuine offer to Royal Mail that would have given space for detailed discussions without a strike," said Dave Ward, deputy general secretary of the CWU.
"We were severely disappointed that within two or three hours the company rejected it, apparently without even affording it proper consideration."
The union has to give one week's notice of when strike action will be taken, so Thursday was the soonest the action could take place.
Last week, members of the CWU voted by three-to-one to support strike action as part of a dispute about pay, modernisation and working conditions.
The union called on the government to intervene in the dispute, especially regarding the pension deficit, which it estimated would exceed £10bn.
"This government has recently intervened in the problems of Vauxhall and Lloyds Bank," Mr Ward said.
"They are the sole shareholder of Royal Mail and it is unbelievable that they continue to stand aside."
Business Secretary Lord Mandelson said unless the union co-operated fully with changes, Royal Mail would go into a "downward spiral".
"I appeal to the union, even at this stage, to draw back and get back to the table, and talk to the management and sort this out properly," he said.
The Royal Mail had more strong words for the union.
"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," Mr Higson said.
"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."