The Government is on a collision course with postal workers after confirming it is pressing ahead with controversial plans to privatise the Royal Mail, sparking claims that it is "selling off the family silver".

Ministers said it was a "very exciting" day as details of the share offer were announced, but they were attacked by unions and opposition politicians.

The scale of anger was highlighted by a noisy reception given to Royal Mail chief executive Moya Greene as she addressed a meeting in Birmingham of more than 1,500 officials of the Communication Workers Union (CWU).

She was booed, heckled and greeted with laughter when she defended the sell-off.

The union will now ballot members for strike action.

Industrial action could start by mid-October, just as potential investors are being courted to buy shares. The sale is expected to take place in November.

Members of the public will be able to apply for shares - the minimum amount being £750 - as well as institutional investors. Analysts expect the sell-off will raise up to £3 billion.

CWU general secretary Billy Hayes said: "This isn't about what's best for the Royal Mail, it's about vested interests of government ministers' mates in the City.

"Privatisation is the worst way to access capital, it's more expensive than borrowing under public ownership."

Chuka Umunna, Shadow Business Secretary, said: "Ministers are pushing ahead with this politically-motivated fire sale of Royal Mail to fill the hole left by George Osborne's failed plan.

"This is taking place despite opposition from a huge coalition, including the Conservative Bow Group, the Countryside Alliance, the National Federation of Subpostmasters, the cross- party BIS Select Committee as well as Royal Mail employees themselves.

"The Government has not addressed the huge concerns which remain on the impact the Royal Mail sale will have on consumers, businesses and communities, but ministers are ploughing on regardless."

Business Minister Michael Fallon said: "It is the final step to help modernise the business and allow it to invest in the future."

The minister said the sale would allow Royal Mail to access capital markets like any other British business.

At least 50% of the business will be sold, although Mr Fallon would not give exact details of the size of the sale and would not estimate how much money will be raised.

Asked about the prospect of a postal strike, he said: "Strike action will not derail the sale."

The Government confirmed that 10% of shares will be given to the Royal Mail's 150,000 workers.

Unite general secretary Len McCluskey said the Government was selling off the family silver, adding: "This move is being driven by blinkered right-wing dogma that has ridden roughshod over public opinion.

"We predict that the sell-off will lead to worse services for business and the general public, particularly in rural areas, as well as job losses, when private companies try to squeeze the last drop of profit out of the Royal Mail."

SNP MP Mike Weir said: "This is a deeply flawed decision, which threatens to have particularly harmful consequences for Scotland, where so many of our rural communities and so much of our rural economy rely on our existing Royal Mail service. The UK Government are refusing to heed the real and genuine concerns that privatisation will mean a reduction in rural services as companies cherry-pick and compete in larger urban areas."

"Given our experience of previous privatisations no one believes that the regulator will provide real protection to consumers on either prices or services."