National Grid Transco, the leading energy and utility company, is outsourcing its IT infrastructure to CSC in a seven-year, £270m deal.
CSC will manage and support National Grid Transco's UK mainframe, application server, desktop, help desk and telecommunications services.
The contract also includes a major infrastructure transformation programme to support more than 18,000 users.
Around 330 National Grid staff will transfer to CSC next month with the outsourcer assuming management responsibility for 170 contractors.
National Grid Transco group director Steve Holliday said, "In addition to providing additional IT expertise and new career opportunities for transferring staff, we anticipate the agreement will result in substantial cost savings."