The Communication Workers Union opposes the plans that would see Consignia outsource its car, van and truck fleets, and said it would ballot its members for industrial action.
The initiative could involve the transfer of 1,700 people employed in its leasing, fleet management and maintenance operations to another employer.
John Keggie, deputy general secretary of the CWU, said the union opposed the plans 'legally, politically and industrially'.
'We have raised our concerns with Government,' said Keggie. 'We have tried without success to convince Consignia not to go down this road, which we believe will be to the detriment of our members and the business.'
Consignia has pledged that staff employment rights will be protected by law under TUPE.
The company has already outsourced its catering facilities in a joint venture in which it retains a 51% share - and says a similar structure would be its preferred fleet solution.
Peter Restarick, Consignia Services Group project director, said the initiative was not merely about saving money on Consignia's £400 million fleet spend, but was driven by the strategy of focusing on its core business - delivering mail.
He added that prospective outsourcing partners will be invited to advance ideas about how they would achieve the balance between improving quality of service (including Consignia's noted environmental commitments) while achieving cost reductions, but insisted that Consignia will not countenance any plans to outsource mail delivery staff.
The company already leases a number of its 4,000 company cars, 29,000 vans, and 7,000 heavy commercial vehicles from lessors including GE Capital, Lombard, Capital Bank, First National, Lloyds, Dresdner and Barclays.
Any decision is not expected until the new year at the earliest.