The contract with Consignia - formerly the Post Office - would be one of the most complex and high-profile outsourcing deals in recent years. The company employs about 2,000 in-house IT staff.
Earlier this year Computer Weekly revealed that Consignia was preparing to outsource its IT and business services. Siemens was tipped as the hot favourite to win the contract.
In a statement this week, Consignia said outsourcing its IT would widen its skills base and free up capital that could be reinvested in the postal service's core services and products.
The company's preferred model for outsourcing is through a joint venture in which it would retain a 51% stake. Given the complexity of the contract, Consignia said it could take up to 15 months before it finds a suitable partner.
Outsourcing the IT department is part of a sweeping review of Consignia's support services. Facilities management, human resources and health and welfare are also to be farmed out.
The deal has already run into controversy. Earlier this year postal workers threatened to take strike action over the outsourcing plans.