According to the Financial Times, up to 90 MPs could vote against the bill when it enters the House of Commons later in the year.

Last week, business secretary Lord Mandelson published a bill that outlined the plans to sell off around 30% of the service – a move he described as the only credible way to save the postal operator.

However, in an interview with The Observer, Mandelson stated that there would be no backtracking from the proposed 30% sell-off, insisting the pension and job security of employees were "in danger" if Royal Mail's finances were not "sorted out".

He added: "Some in the party may be weary of taking decisions, but that simply signals that we're ready for a rest, inviting electoral defeat."

Royal Mail workers have already rallied in Westminster's Central Hall to protest against the planned partial-privatisation of the mail operator, which some say will undermine the service and lead to job losses.

Also see: Major boost for Royal Mail as four divisions post profits