CWU's Dave Ward: "We want an agreement, not a strike"
The union representing Royal Mail workers says national postal strikes could begin on Thursday 22 October.
The Communication Workers Union (CWU) urged Royal Mail to agree to a "peace plan", but said if this failed, they would have "no option" but to strike.
Royal Mail said it was "very disappointed" with the move and argued the union was tabling "fresh demands" rather than proposing an agreement.
Business Secretary Lord Mandelson said striking was a "lunatic proposition".
He said the government was talking to both sides and called on workers to end their proposition of national strike action.
He told BBC News it was a "terrible message" to send to Royal Mail's customers at a time when the organisation should be introducing changes without which it had "a very poor future".
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 action is expected to take the form of rolling strikes, affecting different parts of the organisation on different days, rather than an all-out national strike by postal workers.
Lord Mandelson urged the union to pull back from "a suicidal course of action for Royal Mail"
A number of localised walkouts have already been taking place across the country over the same issues for several months - with more scheduled for London on Friday.
The CWU said it would give notice of the actual national strike dates on Thursday 15 October.
By law, it must wait one week before the industrial action begins - hence the expected start date of 22 October.
Royal Mail said it would respond to the CWU's demands, but added: "If the union are serious about resolving this dispute they should immediately lift the threat of strike action."
"We're very disappointed that the CWU continues to threaten customers with national strike action and still fails to honour repeated offers to call off all strikes even though we told them two weeks ago there would be no further changes this year," said Mark Higson, Royal Mail's managing director.
CWU deputy general secretary Dave Ward said workers did not want to take action, "but neither are they prepared to put up with continuing attacks from a management which is failing".
"We have today written to Royal Mail making it clear that the CWU is ready to issue notice for a national strike as voted for by three quarters of postal workers.
"More importantly, we have offered what we believe is a genuine alternative to reach a lasting agreement. This is an opportunity to avoid a national strike, restore customer confidence and resolve the concerns of staff."
The deal would address issues of job security, work levels, bullying and pay, the union said.
The demands in the letter include:
Royal Mail revealing its long-term business plan
Modernisation changes being agreed rather than imposed by management
Payments for workers being linked to the firm's success
Bullying and harassment allegations being subject to an independent inquiry
The union also said the idea of meeting with third party negotiators to reach an agreement should be considered.
In last week's ballot, 61,623 out of a total of 80,830 workers who voted said they wanted to strike.
But Royal Mail has said 60% of postal workers working in the UK did not vote to strike.
The company and unions have been unable to resolve differences about how best to modernise the postal service.
The union insisted that the company was imposing changes on postal workers, cutting jobs and pay, which they maintained was leading to a worsening service.
Royal Mail denied this, arguing that managers were implementing a deal agreed between the two sides, drawn up after the last national dispute two years ago.
A spokesman for the prime minister said that "a national strike would be completely self-defeating", adding Postal Affairs Minister Pat McFadden was encouraging the two parties to keep talking to each other and to try to settle the dispute.
If postal strikes are confirmed, this would be a "serious knock to business confidence", according to David Frost, director general of the British Chambers of Commerce (BCC).
"The run-up to Christmas is a vital trading period for firms and if this strike goes ahead orders will be lost and the fragile recovery will be undermined," he said.
On Monday, John Lewis announced it was working with other carriers in order to avoid disruption to its online deliveries from a Royal Mail strike.
Royal Mail made an operating profit of £321m in the year to 31 March, but it was the first time in 20 years that all four parts of the business had been profitable.
The company said the number of letters and parcels its core business delivers was falling by 10% each year, losing it £170m per annum.
One major reason for this is increased competition from electronic forms of communication such as e-mail.
Both the Royal Mail and the CWU agree that job cuts are needed as part of streamlining the mail service to cope with the drop-off in postal volume.
But the CWU said Royal Mail managers were refusing to meet its demand for a signed agreement determining the scope of cuts, as well as job and pay security guarantees for those workers who will ultimately remain in their jobs.
The Royal Mail said that by signing up to an agreement after the last national postal strike in 2007, the CWU accepted the future job cuts it needed to make in order to modernise the service.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.