The train operator Eurostar has handed the fledgling agency a brief to develop its environmental strategy.
The agency will also be offered further strategic tasks in the future. The appointment does not affect the ongoing pitch for the main Eurostar account.
The initial brief requires the agency to develop a strategy for the company's environmental credentials into the next decade.
In October this year, Euro-star commissioned research showing how a commercial flight between London and Paris created ten-times more carbon dioxide per passenger than the equivalent trip on a Eurostar train.
Greg Nugent, Eurostar's head of UK marketing, said: "It's a massive piece of intellectual work. Everyone knows Hurrell and Dawson are big future thinkers, so to partner them is a great opportunity."
Neil Dawson, the co-founder of Hurrell and Dawson, said: "Everyone knows that founder clients shape the future of an agency. We couldn't be happier."