Up to 30,000 jobs are to be cut over the next 18 months at Consignia - formerly the Uk Post Office.
It is part of a £1.2 billion (sterling) savings package designed to get the organisation out of the red.
"The job losses will come on top of a reduction of around 10,000 in the organisation's 200,000 staff over the past year," said chief executive Mr John Roberts.
"It is hoped that the bulk of the cuts will be reached through natural wastage, voluntary redundancies and outsourcing of contracts for activities like cleaning and catering."
"Many parcel delivery routes are to be franchised out - often to existing Parcelforce workers - in order to cut costs," he said.
The staffing reductions have been forced on Consignia by tumbling profits, which saw the company record a £281 million loss in the first six months of this year, with operating losses quadrupling to £100 million.
Mr John Keggie, deputy general secretary of the Communication Workers' Union, said the figure of 30,000 job cuts was "completely unreasonable".
He warned: "Any suggestion of compulsory redundancies will lead to appropriate industrial action."
The union revealed earlier this year that Consignia had plans to make cuts of £1.2 billion but it said this would lead to 15,000 job cuts.
PA