As the value of the business soared by more than £1billion in early trading on the London Stock Exchange, Labour's Shadow Minister for Postal Affairs, Ian Murray, MP said: "As the clamour for shares has shown, taxpayers have been short-changed by the Tory-LibDem Government.
"The share offer was oversubscribed by twenty times and the Government received applications worth more than £30 billion for £1.7bn of shares.
"The jump in share price also adds further pressure to the promises of the SNP that it would renationalise the Royal Mail in an independent Scotland - exactly how would they fund the purchase given the value has risen by hundreds of millions of pounds?
"Yet again, Alex Salmond has cynically used this issue to make his false case for separation - a case where he has had no discussions with either Royal Mail or the UK Government and where he can't even tell us the cost of a stamp in a separate Scotland."
Yesterday's huge rise in value at Royal Mail netted big profits for investors but prompted accusations that the Government had sold the business too cheaply.
Nearly 700,000 small investors saw their stakes increase by around £280 as the share price closed up more than a third at 455p.
Billy Hayes, general secretary of the Communication Workers Union, said there would be no celebrations over the share surge at delivery offices, amid fears that privatisation would lead to reduced services and higher prices for consumers.