Dave Ward, deputy general-secretary of the CWU, said: "They are not modernising the service, they are planning huge cuts.
"What we want is an opportunity to resolve this. We don't want to damage customers - we have no alternative."
Gordon Brown, Britain's prime minister, said the strikes would be self-defeating, leading to more job losses.
He said the government would do everything it could to resolve the dispute.
"It's essential that everybody gets round the table. This strike is solvable, and I believe that management and the workforce can reach a solution to this and we should do this as quickly as possible," Brown said.
The government had wanted to sell up to 30 per cent of the company to make it more competitive, but shelved the plans earlier this year due to adverse market conditions and strong opposition from workers and politicians from its own Labour party.
Royal Mail's business has declined by about 10 per cent annually in recent years as customers switch to the internet or more specialised services.
The CWU has blamed Royal Mail bosses and Lord Mandelson, the country's business secretary, for a breakdown in talks aimed at resolving the dispute, saying they sought revenge for the staff's opposition to partial privatisation.
Paul Tolhurst, operations director at Royal Mail, said he wanted the union to sign up to agreements apparently reached this week.
"We are waiting for a call from the CWU, our phones lines are open, our doors are open," he told BBC radio.
"We will meet them as soon as we can. What we've got to get away from is these damaging strikes."
Mandelson said that if Royal Mail does not change, the company faces "terminal decline", but David Cameron, leader of the opposition Conservative party, said the government's failure to go ahead with its part-privatisation plans had galvanised union action.
The CWU said it would announce further strikes next week while Royal Mail has said it will hire 30,000 temporary staff to work on the expected seasonal backlog.