Proposals by the British government to sell part of the Royal Mail are to be introduced in Parliament on Thursday.
Ministers want to sell a stake of about 30% to the private sector to help pay for the modernisation of the service.
The BBC reports the plan faces opposition from a large number of Labour MPs, with 139 signing a motion opposing any sale and from unions who plan a protest on Tuesday.
Both the Conservatives and the Liberal Democrats have backed the move in principle.
In December, Business Secretary Lord Mandelson endorsed the recommendations of an independent report which called for a minority stake in the business to be sold.
He said Royal Mail could not survive in its current form, due to severe financial constraints and falling demand for sending letters, and needed fresh investment in technology to prosper.