Postal workers repeated their demand to keep the post public on Monday after reports suggested Royal Mail might be sold off next year.
A share offering, or IPO, for the vital 500-year-old service would be the biggest privatisation since Britain's railways were handed to privateers in the '90s.
But the Communication Workers Union vowed to fight any sell-off attempt, saying it wouldn't be in the interest of "customers, postal services, the company or the people who work for it."
And last week Unite national officer Ian Tonks said the union "remains firmly opposed" to Royal Mail privatisation.
"Privateers are not interested in providing a public service, they are only interested in making a profit at the expense of consumers and our communities," he said.
The Post Office would remain under state control but would probably be made into a mutual company.
Although the news filtered out on Monday, observers have been expecting developments for days.
Royal Mail's £9.5 billion pensions deficit was taken on by the government last week, which offered more security to workers but was also a clear step towards selling it off.
Communications regulator Ofcom has already tried to make Royal Mail more attractive to buyers with proposals to let it set most of its own prices.
The government is reported to be looking at a number of options for selling off the whole or part of the company, but putting a price tag on it is hard - estimates range from £3bn to £5bn.
Its letters business has been hit hard by internet communication but it still made a £39 million profit in 2010-11 on sales of £9.2bn.
Royal Mail goes back to 1512, when a royal official called Brian Tuke was made "master of the posts" by Cardinal Wolsey. It expanded across the country and was once Britain's largest employer.
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