The move comes after a six-week review sparked by the poor performance of Bates' Cordiant parent. The ownership of Cordiant has been in contention for several weeks.
JWT's parent, WPP, is poised to acquire Cordiant. However, this is thought to have had only a limited influence on the appointment of JWT, as talks between the agency and B&Q pre-date those between WPP and Cordiant.
JWT is expected to accommodate some of the account management and creative teams that worked on the B&Q account at Bates.
The win comes shortly after JWT picked up the £23 million AXA account and it is an important addition to the agency's domestic client list.
Speculation surrounding the future of the B&Q account had been mounting following the news last month that the retailer had opened discussions with rival agencies. These discussions are said to have taken place with several shops, including M&C Saatchi, Saatchi & Saatchi, Publicis and DFGW.
The strong relationship between Bates and B&Q has produced successful campaigns over the years. Since the 80s, B&Q has risen to fame with its red apron-wearing staff and the "you can do it if you B&Q it" slogan.
The sheer size of the account contributed to Bates' strong retail credentials throughout the 90s.
B&Q is the latest in a string of major advertising contracts to be lost by Bates UK as confusion has mounted about its future ownership. Woolworths shifted its account to Bartle Bogle Hegarty and Royal Mail moved to Abbott Mead Vickers BBDO earlier this year. Another Bates client, Sky, moved its multimillion-pound Sky Plus account to HHCL/Red Cell following a pitch in March.
B&Q is one of the largest DIY retailers in Europe. It is the UK's 33rd-largest advertiser, according to Nielsen Media Research, and produces more than 200 UK-specific ads each year.
JWT was unavailable for comment as Campaign went to press.