Work will now begin on transferring the Government Gateway, which is central to minsters' e-government drive, by allowing legacy systems in different departments to communicate.
Other services being transferred to the datacentre include the Knowledge Network information sharing facility and Delivering on the Promise, a pan-government content management system.
ITNet will be the sole service provider, replacing troubled telecoms giant Cable & Wireless and managed services provider Loudcloud, which last month sold its global managed services business to system integrator Electronic Data Systems (EDS) for £43m.
The initial tender document published last year called for a minimum of two data centres with top-level security and protection to run the service.
A Cabinet Office spokeswoman said that the government would undertake a phased approach to transferring the three services, the first of which will be completed early next year.
"We will be conducting a phased transition of the three infrastructure building blocks - the first of these phases is due to be complete in early 2004," she said.
Douglas Alexander, minister for e-transformation at the Cabinet Office said, "The government has a target to get all services online by 2005, achieving high levels of use in key services. Resilient, flexible solutions like the Data Centre Hosting project will provide the fundamental infrastructure to help us achieve this."
Although the value of the initial five-year contract is £83m, there is an option to extend this for a further two years.