Unions representing postal workers and managers, and the Labour Party, criticised the coalition for pressing ahead with the sell-off despite "consistent opposition" from the public and warnings of the impact on jobs and services.
The Government said Royal Mail expected postal workers to vote for a national strike and other forms of industrial action.
Shares are expected to be priced at between 260p and 330p when privatisation takes place next month, giving a market valuation in the range of between £2.6bn and £3.3bn.
Unconditional dealings in the stock will start on October 15, the day before voting closes in a ballot by the Communication Workers Union (CWU) over whether to back a campaign of industrial action on the sale. The union is urging its members to vote yes, with any strike action starting a week later.
Around 150,000 eligible staff will be handed 10% of shares in the largest free stock offer of any privatisation in the UK over the past 30 years. This is equivalent to around £2,000 each at the mid-point of today's offer price range.
Billy Hayes, CWU general secretary, said: "It seems remarkable that the prospectus is being issued on the same day that postal workers are being sent ballot papers for strike action. Today's announcement changes nothing in terms of the ballot, which will go ahead as notified.
Business minister Michael Fallon said "We are not selling the Queen's head and we are not selling the post offices. We are selling Royal Mail. The six-days-a-week universal one-price-goes-anywhere service is absolutely separate."