A late rush to buy shares in Royal Mail is under way in Britain, with investors predicted to receive an instant profit of up to 40 per cent.
Stockbrokers were preparing to open late on Monday and Tuesday - the final day for applications - to cope with demand before the biggest government flotation for two decades.
The late surge for shares comes as ministers have been accused of offloading the mail service on the cheap, enabling City speculators to make millions from the selloff.
Royal Mail's shares are priced between £2.60 and £3.30, but are expected to be valued far higher than that when trading starts next week. Analysts at Panmure Gordon said the company was worth up to £4.5 billion ($7.6 billion), well in excess of the upper valuation of £3.3 billion.
Investors can apply for up to £10,000 worth of shares and the flotation is expected to be oversubscribed, as were many of the British privatisations in the 1980s.
The popularity of the sale has sparked a political row, with the Labour opposition claiming it should be halted because the massive demand shows Royal Mail is undervalued.
Claims that the company is undervalued are good news for private investors and are likely to be even better news for City investors and hedge funds.
The close of the share offer on Tuesday will be followed by a three-day period of "conditional trading", when City institutions can buy and sell the shares between each other.
The final price will be announced on Friday and shares in Royal Mail will begin trading on the London Stock Exchange from Monday.
Labour said it was concerned that hedge funds and speculators would make millions before the shares were available on the open market.
Labour MP Alan Johnson, a former business secretary, said: "There is a vast difference between pricing Royal Mail shares conservatively and undervaluing them by £1 billion. This is ripping off the taxpayer on an epic scale."
Party sources suggested Labour was looking at ways to use parliamentary procedures to delay the float and allow a rethink on the valuation.
Part of Labour's concern is the government's City advisers have failed to take into account the £1 billion-plus property value of some of the Royal Mail's sites, particularly the Mount Pleasant and Nine Elms delivery offices in London.
If Royal Mail decided to sell the sites, any profits would be shared by the company's shareholders, rather than with the taxpayer.
A cabinet minister dismissed the fears and said if the shares were oversubscribed it was "a triumph".
A spokesman for the Business Department said: "The government is approaching the sale as a commercial shareholder and is seeking ... the best deal for taxpayers.
"This is a commercial transaction and government is following normal commercial practice in setting and publicising the share price and delivering value for the taxpayer.
"The value of Royal Mail will depend on a number of factors, notably the company's ongoing financial performance, its future prospects and the level of investor interest. The government will hold on to a stake in the business which could be sold at a future date."