Fujitsu will supply the specialist skills required to manage the legacy applications that support British Gas's residential and business customers.
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Centrica also expects cost reductions of between 10% and 20% from Fujitsu's applications value assessment programme.
Under the programme, mature applications will be redesigned to run at lower cost or replaced by more cost-efficient software.
Neil Coop, senior commercial manager at Centrica said the energy company had extended its 30-year relationship because of Fujitsu's skills and resources to continue helping Centrica to modernise its legacy systems.