The move mirrors rival HOK's alliance with four European firms to share expertise and local knowledge, which was announced last month.
Cathy Tondelli, WATG's business development director, said this week that its British clients were benefiting from the range of skills on offer. Although they are not merging, the firms have agreed to work on projects together, share research and co-ordinate marketing.
Tondelli said the alliance was working on three projects around the world.