Capita has been awarded a five-year contract worth £5.7m with Essex Partnership University NHS Foundation Trust (EPUT).
The deal will see EPUT move to the Health and Social Care Network (HSCN) which is a new data network designed to replace the NHS' legacy N3 model.
The contract will cover 400 EPUT sites - including hospitals, GP surgeries and NHS clinics - and Capita claims it will provide the trust with increased bandwidth as well as efficient access to the internet and N3's legacy applications.
Mark Madden, CFO at EPUT, said: "We need a reliable and robust connectivity solution to deliver enhanced connectivity.
"We are confident that this implementation will enhance the user-experience for our employees and enable greater interaction and sharing of resources between our sites in the Essex region."
The contract is the largest HSCN contract to date.
Joe Hemming, executive director at Capita IT and networks, said: "Capita is an experienced provider of IT networks and this contract further reinforces our strategy.
"We look forward to using our technology capabilities to add value and efficiencies for NHS sites across the region."
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