The sell-off, of 15 per cent of the business, is expected to raise £767m for the government. The shares are likely to be priced tomorrow, while the settlement date will be on 15 June.
"The business secretary has today authorised a sale process to begin. Current market conditions represent a good opportunity to achieve value for the taxpayer," a Department for Business spokesperson said.
"The universal service is strongly protected by law and Ofcom has a duty to ensure its provision. Therefore the Government sees no policy reason to retain a stake in Royal Mail."
Chancellor George Osborne has previously said the government has appointed Rothschild to advise the Department for Business, Innovation and Skills on the sale of its remaining 30 per cent stake in Royal Mail.
"It is the right thing to do for the Royal Mail, the businesses and families who depend on it - and crucially for the taxpayer," Osborne said at the time.
Shares in the business were first floated back in October 2013 at 330p - but they last closed at around 516p.