Ministers are pressing ahead with the sell-off despite opposition from postal workers and other groups.
Labour accused the government of putting party politics ahead of the interests of taxpayers by rushing to sell off Royal Mail “on the cheap”.
Shadow business secretary Chuka Umunna said: “As we have seen in the rail and energy industries, badly-executed privatisations result in sub-standard services and people being ripped off. That is why any government intent on doing a sell-off should exercise care, rather than rushing to do a sell-off in this way.”
Billy Hayes, Communication Workers Union general secretary, said: “Now isn’t the time to privatise Royal Mail. We don’t want our prize assets to be flogged at bargain basement prices just to cover up George Osborne’s mess. This government has failed time and again to get value for the taxpayer.”