The system, which allows government employees and suppliers to exchange e-mails over a secure network, is now being run by Energis, after the networks firm won the contract from the incumbent Cable & Wireless last August.
The GSi, which uses datacentres and applications supplied by Fujitsu, will give government employees restricted-level access to enhanced e-mail and internet browsing as well as managed remote working.
The system, purchased by OGCbuying.solutions, an agency of the Office of Government Commerce, will be developed to link the criminal justice network, the NHS network, the Ministry of Defence and the European Union with more than one million potential users.
Hugh Barrett, chief executive of OGCbuying.solutions, said the upgraded GSi represented a "step change" in the government's use of communication technology.
"The new contract, which is available to central civil government and the wider public sector, as well as government suppliers, has the potential to finally deliver joined-up government in a way not previously possible," he said.
Energis already manages 2,000 government sites on its network, including the Crown Prosecution Service, the Employment Service, Inland Revenue and the Vehicle Inspectorate.