The bill was published this morning by business secretary Peter Mandelson, starting a legislative timetable that envisages the sale of up to 30% of Royal Mail by the summer.
Billy Hayes, leader of the CWU, said the bill offered "no convincing argument for privatisation".
He said: "It's also clear that there's no support among Labour MPs for this move, which is why Lord Mandelson introduced it in the House of Lords."
More than 130 Labour MPs have signed an early day motion against the part-privatisation and the Financial Times claims they include 11 ministerial aides who have not been disciplined as a result.
Mandelson promised that the network of post office branches would remain 100% publicly-owned and no private company would be able to take majority shareholding without new primary legislation going through parliament.
In addition, the government will set up a fund to ensure Royal Mail's six days per week universal service will continue and will take over the bulk of the company's pension liabilities.
These cover the £6bn in liabilities incurred before December 2008, with all future pension liabilities remaining with Royal Mail.
Mandelson said: "Our policy will keep Royal Mail in the public sector and this legislation makes this clear. The Royal Mail will run out of money to sustain its current universal, six-day service unless its pension fund deficit is solved and its business transformed."