The union representing more than 100,000 postal workers have called on Business Secretary Vince Cable to resign as the row over the Government's controversial privatisation of the Royal Mail intensified.
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: "On the opening day of the flotation Vince Cable wrote off the undervaluation as froth. A week later, we were told it was the fault of the CWU.
"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. We would also like the matter to be referred to the Public Accounts Committee to scrutinise how badly the taxpayer has been left out of pocket."
Shadow Business Secretary Chuka Umunna said: "Vince Cable has said that taxpayer value was 'central' to the Government's strategy in selling Royal Mail but given the extensive consultation with institutional investors and banks which took place, both he and the Prime Minister have serious questions to answer.
"Crucially, they must explain when the Government was made aware that the sale was so massively oversubscribed by major investors and why, having considered a higher price, they rejected that option."
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.
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."