Ministers said it was a “very exciting” day as details of a share sale were announced, but they were attacked by unions and opposition politicians.
Members of the public will be able to apply for shares - at a minimum of £750 - as well as institutional investors.
Analysts expect the sell off will make up to £3 billion.
The announcement came ahead of a meeting of Communication Workers Union officials to discuss the sell off and their plans for a strike ballot.
General secretary Billy Hayes said: “This isn’t about what’s best for the Royal Mail, it’s about vested interests of government ministers’ mates in the City.
“Privatisation is the worst way to access to capital as it’s more expensive than borrowing under public ownership.”
Chuka Umunna, shadow business secretary said: “Ministers are pushing ahead with this politically-motivated fire sale of Royal Mail to fill the hole left by George Osborne’s failed plan.
“This is taking place despite opposition from a huge coalition including the Conservative Bow Group, the Countryside Alliance, the National Federation of SubPostmasters, the cross party BIS Select Committee as well as Royal Mail employees themselves.
“The Government has not addressed the huge concerns which remain on the impact the Royal Mail sale will have on consumers, businesses and communities, but ministers are ploughing on regardless.”
Business minister Michael Fallon said: “It is the final step to help modernise the business and allow it to invest in the future.”
The minister said the Government was aiming for a majority sale so that Royal Mail could access capital markets like any other British business.
At least 50% of the business will be sold, although Mr Fallon would not give exact details of the size of the sale and would not estimate how much money will be raised.
He said there was “sufficient appetite” from investors to justify going ahead with the sell off.
Asked about the prospect of a strike by postal workers, he said: “Strike action will not derail the sale.
“There is no need for strike action - a pay rise of 8.6% over three years has been offered. Teachers and nurses are only getting 1%.”
The Government confirmed that 10% of shares will be given to 150,000 Royal Mail workers.
Unite general secretary Len McCluskey said the Government was selling off the family silver, adding: “This move is being driven by blinkered right-wing dogma that has ridden roughshod over public opinion which is strongly opposed to the sell-off of this national institution.
“We predict that the sell-off will lead to worse services for business and the general public, particularly in rural areas, as well as job losses, when private companies try to squeeze the last drop of profit out of the Royal Mail.”