Alongside its existing creative remit on the retailer's brand, BBH will now be responsible for Tesco’s more price-driven trade advertising, which was previously handled by Tag.
The move significantly reduces Tag’s account, although it will still work with Tesco on creative production work including point of sale and web production activity.
A Tesco spokesman said: "Tag will continue to provide creative production services to Tesco and remain a valued supplier."
Meanwhile Tesco is "exploring options" on other parts of its marketing activity.
A Tesco spokesman said: "We’ve recently restructured our marketing team and are currently exploring options in some areas. Havas Helia will continue with their work on CRM."
In his original statement on the BBH appointment, Tesco chief executive, Dave Lewis, said BBH would focus on "advertising and customer relationship marketing".
But yesterday the Tesco spokesman said: "I can confirm there will be no changes to CRM arrangements for at least the next six months."
When Tesco shifted its £110 million ad account to BBH without a pitch in January, it rejected suggestions that Tag or Helia would lose their business as a result.