Work will now begin on transferring the Government Gateway, which is central to ministers' e-government drive, by allowing legacy systems in different departments to communicate.
Other services being transferred to the datacentre include the Knowledge Network data-sharing facility and Delivering on the Promise, a pan-government content management system.
ITNet will be the sole service provider, replacing Cable & Wireless and Loudcloud.
A Cabinet Office spokeswoman said the government would undertake a phased approach to transferring the three services, the first of which will be completed early next year.
Although the value of the initial five-year contract is £83m, there is an option to extend this for a further two years.