The government has been accused of threatening the universal postal service after announcing plans to sell its remaining 30% stake in the Royal Mail.
Chancellor George Osborne said sales will start this year, with the expectation of raising around £1.5 billion.
The move will complete the controversial privatisation of the postal group, which started in October 2013.
The coalition was attacked for selling off Royal Mail "on the cheap", and there was fresh opposition to today's announcement, including the threat of industrial action.
The Communication Workers Union (CWU) said it will oppose the final sale, warning that full privatisation threatened the one-price-goes-anywhere, six day a week postal service.
Labour said the previous sell-off had been "botched", short-changing taxpayers by hundreds of millions of pounds, so the Government needed to explain how it had learned lessons.
The Government has appointed Rothschild to advise the Business Department on the new transaction, which will be designed to deliver "best value for money" to the taxpayer.
Shares were initially floated at 330p each, but leapt by 38% on the first day of trading, peaking at 615p
Opponents of the sell-off have claimed it was set as much as £1 billion too low but a report commissioned by former business secretary Vince Cable from Lord Myners said the figure was £180 million.
CWU general secretary Dave Ward said: "Selling off the final 30% of Royal Mail threatens the very existence of the one-price-goes-anywhere, six day delivery service that Royal Mail provides to 29 million UK addresses.
"When the first part of this privatisation was completed by the coalition we were told that this was because Royal Mail needed private capital to invest in its future, but if you ask the workforce they have seen hardly any new investment but have witnessed the worse type of short term investors making a killing without any regard to the long term future of the company or the services it provides to the public.
"The CWU will oppose this final part of the sell-off and continue to campaign against unfair competition and the race to the bottom, which privatisation inevitably brings. Both existing and any new shareholder should be in no doubt - any attempt to undermine the legally binding agreement that protects Royal Mail workers' terms and conditions will be defended if necessary through strike action."
Shadow chancellor Chris Leslie said: "The Tories' botched privatisation of Royal Mail in the last Parliament short-changed taxpayers by hundreds of millions of pounds.
"The Government now needs to explain how it has learned the lessons of last time so that the same mistakes aren't simply repeated."
Shadow business secretary Chuka Umunna said: "Having short-changed taxpayers by hundreds of millions of pounds when initially privatising Royal Mail, ministers are now selling off the public's remaining stake in the business.
"At the time, they promised Royal Mail would be sold to long-term investors, but instead City speculators took the lion's share and made a killing, with shares being sold off immediately.
"With vital postal services already being put in jeopardy as a result of the Government's fire sale, the risk is that this move will further weaken their future. Consumers and businesses which rely on Royal Mail need reassurance that essential services like daily deliveries and the availability of Royal Mail services through the Post Office network will continue."
Brian Scott, of the Unite union, which represents Royal Mail managers, said: "Selling off the last remnants of the family silver to pay for self-defeating austerity will further undermine the future of the universal postal service.
"It is likely that this will be another mishandled sale and we would urge the Government to think again and retain its stake in Royal Mail.
"The recent demise of Whistl, despite cherry-picking mail deliveries and ignoring over 50% of the country, demonstrates that the delivery of letters to homes and to businesses is a natural monopoly and one which Government has a responsibility to maintain."
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