The deal involves work carried out by TNT Post's 48-hour Premier service for pre-sorted mail with the supermarket giant continuing to use Royal Mail to carry out the final sort and delivery to customer's homes.
Nicola Sharpe, Sainsbury's spokeswoman, said: "Customer retention is vital in our business and therefore communication is a key part of our success. We chose TNT Post based on high recommendations, coupled with the range of mailing solutions they had to offer.
"We have been particularly impressed by TNT Post's innovative and flexible services, and are confident our postal communications are in safe hands."
Nick Wells, TNT Post chief executive, added: "As competition hots up between the big four supermarkets, Sainsbury's is looking to put itself ahead of the game by choosing a modern mailing solution that will bring benefits both to its business and its customers.
"Our advanced technology and premium services offer Sainsbury's a more efficient and cost-effective mailing solution, keeping costs to a minimum."
This is the latest high-profile win for TNT Post with other clients including Next, T-Mobile, Virgin Mobile, BSkyB, Barclays and Telewest.
In May this year, the Greater London Authority renewed its contract with the mailing firm for door-drop distribution rights for its free newspaper The Londoner, in a deal worth £1.3m.
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