The deal, which runs through to 2010, will provide the infrastructure for the Post Office card account and access to basic bank account services through the Post Office network.
Fujitsu Services, formerly ICL, already provides counter systems in all 17,500 UK Post Office branches as part of the Horizon project.
"This new contract will make life easier for our customers, offering them new services and more choice in how they can pay," said Post Office IT director Alan Barrie.
"Our aim is that this will also make life easier for sub-postmasters by reducing paperwork and giving them the latest information on Post Office products and services at their fingertips."
The UBS project means customers will be able to continue collecting their benefit payments in cash from Post Office branches after April's move from paper-based processes to the direct payment of benefits into bank accounts.
High-street banks will be able to offer deposit and cash withdrawal facilities through the Post Office branch network.
Last year, the Post Office signed a landmark outsourcing contract with EDS to build and support the basic bank accounts for receiving benefits as part of the UBS project.