Brunswick had held the Kingfisher brief, which includes corporate brand strategy, comms planning, financial PR, internal comms and crisis management, for several years. Its chances of holding onto the account appeared to have been boosted when Catriona Woolner-Winders, a former director at the agency, became Kingfisher's group comms chief in August.
The retailer declined to comment.
Kingfisher is the parent firm of B&Q, Screwfix and others - it operates more than 1,100 stores and various online channels in 10 European countries, with 74,000 employees. Adjusted pre-tax profit in 2015/16 was £686m, from total sales of £10.3bn.
In January, it outlined a five-year transformation plan, named One Kingfisher, with the goal of delivering an additional £500m of profit annually by the project's end, led by a six-point list of guiding principles.