Insatiable City traders are gobbling up Royal Mail shares in expectation of a profit bonanza on the undervalued service.

At midnight tonight the feeding frenzy will draw to a close, signalling the end of public ownership for the world's oldest postal service.

Big-fish investors such as City banks and hedge funds have been promised 70 per cent of the shares available,  while the remaining 30 per cent have gone on sale to the public.

Demand is so high for the shares, which are predicted to produce an instant profit of 40 per cent amid reports that Royal Mail and its property portfolio have been undervalued by £1.2 billion, that stockbrokers are staying open late to cope.

CWU general secretary Billy Hayes took the news that Royal Mail has been undervalued as a given.

He said: "It's no surprise that the government has sold the postal service on the cheap to its friends in the City.

"The public didn't want the service sold and will get a worse service as customers.

"Now they also know they are getting as bad a deal as taxpayers too."

The union will ballot members to strike, prompting Royal Mail to offer £300 to employees who do not take industrial action.

Meanwhile Labour are asking for the "plug to be pulled" on the sale at the last minute, though they have failed to commit to renationaling the service in the wake of a 2015 general election victory.

Such a statement would effectively scupper the Tory sell-off by frightening predatory investors away.

Shadow business secretary Chuka Umunna said: "It is not too late to pull the plug on this privatisation. Pull the plug on it.

"But at least if they are going to proceed ensure you get good value for the taxpayer and increasingly we are seeing more and more information coming out which would suggest that the taxpayer will be short-changed."

Cat Hobbs from the We Own It campaign branded the sale as nothing less than theft.

She told the Star: "They've taken what we own away from us. It's astounding."