Vince Cable has been urged to resign by the Communication Workers' Union (CWU) after it emerged that the Royal Mail had been valued at £9.95bn ahead of its stock market flotation, nearly three times the £3.3bn finally chosen.
CWU general secretary Billy Hayes said: "We now have a prima facie case of a conspiracy against the UK taxpayer who were opposed to the sale and have now been robbed of billions.
"In any other walk of life this would be a sacking offence and we call on Vince Cable to resign. A full inquiry should be launched into the mis-handling of this unnecessary privatisation by Vince Cable.
Wall Street bank JP Morgan gave the Royal Mail a potential valuation as high as £9.95bn, Sky News reported.
TOP STORIES TODAY
Despite Royal Mail shares soaring 50% on their first day of trading, the Business Secretary played down suggestions that it had been undervalued as "froth".
Ministers are selling up to 62% of the business, with 10% going for free to Royal Mail staff. David Buik, from City analysts Panmure Gordon, told the Huffington Post UK: "You have to make it attractive to an investor to plough in, if you want them to plough in, you have to make it an absolute bargain."
Last week, the CWU union confirmed it would hold the first national postal strike in nearly four years on November 4 over issues linked to the part-privatisation, which will see a 24-hour stoppage planned on that date.
The CWU ballot of around 115,000 members at Royal Mail came back with a 4-1 decision in support of industrial action, with a 63% turnout.