BBH was appointed by Microsoft in December 2001 and until this week was the only non-McCann agency used by the software giant. The business, worth £4 million in the UK, changed hands without a pitch.
The move is the last stage in the realignment process and follows Microsoft's decision to move the creative account for its non-Xbox business out of Euro RSCG Wnek Gosper into McCann-Erickson in 2001. In the same year, it also awarded its entire media account to Universal McCann.
The decision to cut ties with BBH follows the recent departure of the Xbox vice-president for Europe, the Middle East and Africa, Sandy Duncan.
Since then, Microsoft has looked to wind down Xbox's position as a standalone brand, preferring to align it with its parent company.
The partnership between BBH and Microsoft produced a string of awards including a Cannes gold in 2002. The agency's most famous work for the brand included "Champagne", a viral ad, which was banned from TV following 136 complaints to the Independent Television Commission, and "mosquito", a critically acclaimed cinema execution, written by the then BBH creatives Fred and Farid.
Microsoft launched the Xbox in March 2002, retailing at £299. There then followed a string of price cuts in response to disappointing sales. As of last month, the product became available for £129.99, which is the same price as the Nintendo Game Cube - its main rival for second place in the console market behind the Sony PlayStation 2.
The win marks the third time McCann has leveraged significant extra business out of existing clients this year. The agency was handed the £5 million brief for Vanilla Coke in March and has since landed the £12.5 million L'Oreal Plenitude business. It also won the global creative account for Capital One last month.