The sale of a majority stake in Royal Mail was due to be announced on Thursday, the BBC reported, sparking an angry response from unions.
The government is expected to announce its intention to float the 497-year-old service before the London Stock Exchange opens on Thursday, according to the report.
The National Federation of SubPostmasters (NFSP) warned that the £3bn sale would be a "reckless gamble" if it failed to protect post offices.
The Communication Workers Union (CWU) is currently balloting members on strike action.
Business Secretary Vince Cable earlier revealed plans to allow members of the public to buy shares in the postal service with 10 percent of stocks set aside for Royal Mail employees.
The government insists the sale is essential for the survival of the service.
"Parliament decided over two years ago that selling shares...was the right thing to do to secure Royal Mail's future and protect the six day a week, one price goes anywhere, universal postal service," a Department for Business spokesman said in a statement Tuesday.
"A successful, financially sustainable Royal Mail with access to private capital is in the best interests of the workforce and all users of the universal service," he added.