The stock sale would allow Royal Mail to tap investment it needs to modernize and to compete, particularly in parcel delivery, a major source of income as letter use declines in favor of email.
But postal unions and the government opposition have condemned the initial public offering as a "fire sale" of a lucrative state asset. Stock trading starts on Oct. 15 a day before the Communications Workers Union strike ballot closes.
Union workers stand to benefit from the stock sale, with 10 percent of the shares made available for free to 150,000 U.K.-based Royal Mail employees. But there are fears of potential job losses.
"Those who want to sell off the Royal Mail Group are motivated purely by short-term gain and vested interests," the postal union told its members in a statement. "We cannot give the company a free hand to determine your future and the future of U.K. postal services."
The sale will close on Oct. 8, with the actual pricing expected to be announced on Oct. 11. The government's holding is expected to be between 37.8 percent and 49.9 percent.