It comes amid reports that banking giant JP Morgan told the Government earlier this year that Royal Mail was potentially worth up to £9.95 billion.
The Communication Workers Union said it was clear the business had been undervalued, "robbing" taxpayers of billions of pounds.
The union, which is due to stage a strike next month over issues linked to the sell-off, also pressed for the Public Accounts Committee to scrutinise the sale.
General secretary Billy Hayes said: "A full inquiry should be launched into the mis-handling of this unnecessary privatisation by Vince Cable."
Mr Cable dismissed claims that the company had been sold off too cheaply.
He told Sky News: "To be frank, we are not interested in the spivs and speculators, we are interested in serious long term investors."