National Grid has outsourced its finance and human resource support to Tata Consultancy Services with about 300 workers affected.
Late last year National Grid completed a review of its shared services at offices in Warwick, Newcastle and Northampton.
The review included contributions from suppliers. In October last year National Grid proposed transferring some services to TCS.
It also said an office in Newcastle was likely to close and about 300 people would have to look for alternative positions within the company.
TCS was chosen because it had the right track record and quality of services and solutions, it displayed strategic and partner compatibility and commercially was cost effective, according to National Grid.
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Steve Lucas, National Grid finance director, said, "This is much more than 'just' a technology solution. TCS showed a very strong understanding of our requirements and needs and this was crucial to us."
TCS has provided IT services to National Grid for 15 years.
Regarding the affected workers, Steve Noonan, director of UK Shared Services at National Grid, said in October: "We recognise that these are sensitive and difficult times for those employees affected by the result of the review.
"However, National Grid has a great track record in looking after its employees through change. We strongly encourage staff to take up positions elsewhere in the company and to investigate retraining for other available roles.
"And for those who choose to leave the company, we offer generous severance packages, and we provide outplacement support to help them find alternative work."