W&K won the £110 million business after a pitch against TBWA\Manchester and VCCP. JWT London had earlier pulled out of the contest.
Christie described the appointment as an opportunity to work "with one of Britain's truly iconic brands". The work is expected to begin after October, until when the incumbent, The Red Brick Road, has a contract.
Sources close to the pitch suggest that the agency will continue to create work based around the long-running "every little helps" line. One source said that agencies were asked whether they would change the line but then received indications from the client team that it was a key part of the Tesco heritage.
W&K has been retained as the "lead creative and strategic agency". However, Campaign understands W&K has partnered with Tag, which will handle trade and tactical ads.
The Tesco business accounts for around two-thirds of The Red Brick Road's billings, but the agency said it was too soon to put a figure on how many staff would transfer with the business to W&K.