Ministers have been accused of rushing to sell Royal Mail "on the cheap" as the firm is set to rake in billions from selling a number of mail centres and delivery offices.
Members of the Communication Workers Union are currently voting on whether to strike over issues linked to the privatisation.
The result of the strike ballot is set to be announced later this month, coinciding with the expected £3.3 billion flotation of Royal Mail after the government sold 62% of the business.
Among the Royal Mail's property portfolio is Mount Pleasant Centre in North London, which will be redeveloped into luxury homes and could generate up to £1 billion.
The Nine Elms site in South London is estimated to be worth £500 million, and a third site is set to be sold in Paddington, West London, while other sites have been identified as development "potential".
Shadow business secretary Chuka Umunna warned that many delivery centres and mail centres, through which Royal Mail operates, could be sold after privatisation and leave the taxpayer "short changed" and customers could be forced to travel to remote delivery offices.
The Royal Mail privatisation prospectus failed to specify which of Royal Mail's other sites across Britain could be sold off and for how much money, Labour said.
Umunna said: "David Cameron's fire sale of Royal Mail is bad for consumers and bad for businesses and there are real fears that taxpayers are going to be considerably short changed.
"Royal Mail has a huge property portfolio in prime development sites in London and across Britain and there is nothing to stop the privatised company making a quick buck by flogging off these assets for development. Ministers need to come clean on which sites are due to be sold and what valuation has taken place.
"At a time when hardworking people are suffering from a cost of living crisis, David Cameron reveals how out of touch he is, prioritising the fire sale of this cherished national institution to fill the hole left by his failed economic plan."