Some 150,000 Royal Mail workers are to start receiving details of the offer of free shares in the company under the Government's privatisation plan.

The controversial sell-off has set ministers on a collision path with the unions, with the possibility of strike action just as potential investors are being courted to buy a stake in the business.

The free shares offer makes 10% of Royal Mail available to employees at the time of flotation, but the sweetener has failed to convert opponents to the scheme. Information on the free scheme, together with details of a priority shares offer for staff wanting to buy a larger stake in the firm, is due to drop through the letterboxes of eligible workers.

Royal Mail Group's chief executive officer Moya Greene said: "Our people are the heart of our business at Royal Mail. By owning 10% of the company, together we will have a meaningful stake in the business. I think this will engage everyone and encourage us to continue to work together to build a great future for Royal Mail."

Ministers said the flotation would allow the business to modernise, but they were attacked by unions and opposition politicians, with claims that the Government was "selling off the family silver".

The scale of anger was highlighted by a noisy reception given to Ms Greene as she addressed a meeting in Birmingham of more than 1,500 local 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 press ahead with balloting its members for strike action in protest at issues linked to privatisation. 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 to capital as it's more expensive than borrowing under public ownership."

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

Shadow businesss secretary Chuka Umunna 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."

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