Energis replaces incumbent C&W in running the system that enabled civil servants to send e-mails across a secure environment.
The new service is based on an internet protocol VPN. Hugh Barrett, OGCbuying.solutions chief executive, said, "At the end of a hard-fought competition, Energis' offer for the GSI gave us best value for money and an innovative approach."
More than 200,000 civil servants already use the system and Energis expected this to grow to more than 300,000.
Energis will link all central and local government departments to a "next-generation" GSI, as well as provide secure interfaces to other European government networks.
Energis will also use a range of access methods to drive down connectivity charges. Other improvements will include directory services, offering contact details for every individual and department, and a web portal providing secure access to services.
The company will also build a separate, lower cost, secure but unrestricted intranet service to attract government departments not yet using the GSI and have no need for a permanently restricted environment.
Energis already manages 2,000 government sites on its network, including the Crown Prosecution Service, the Employment Service, Inland Revenue and Vehicle Inspectorate.