The National Audit Office (NAO) disclosed 12 priority investors sold all or some of their holdings within the first few weeks of trading.
Critics of the privatisation said the spending watchdog offered “startling proof” the government sold off the country’s family silver “on the cheap”.
However, Liberal Democrat Business Secretary Vince Cable said the report showed the government secured the future of the service through a successful sale.
On the first day of trading last year, Royal Mail’s shares closed at 455p, 38 per cent higher than their price sale – a first day increase in value of £750 million for the new shareholders.
Amyas Morse, head of the NAO, said: “The department’s approach was marked by deep caution, the price of which was borne by the taxpayer.”
However, Mr Cable said: “We secured the future of the universal postal service through a successful sale of a majority stake in Royal Mail, predominantly to responsible long-term investors.”