B-M has already worked on projects for the company over the past few months, and the agency also has Thames Water's government relations account.
B-M will undertake work including drafting think pieces, advising on stakeholder dialogue and training, as well as advising Thames Water in the run-up to the World Summit for Sustainable Development, which takes place later this month in Johannesburg.
Nick Bent, a former researcher for MP Paul Boateng and associate of the think tank Demos, will head the account for B-M, reporting to Sandy Henney, group head of corporate affairs at Thames Water.
Thames Water is the world's third-largest water company, and operates in 46 countries across the world, as well as supplying London with water.
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