The company, which was sold for an undisclosed sum, moves under the wing of Omnicom's Diversified Agency Services division. The deal culminates negotiations that began 18 months ago.
DAS has acquired 100 per cent of the shareholding in Hall & Partners Group and its operating subsidiaries.
Mike Hall, the former Leagas Delaney planning director who formed the company 11 years ago with six staff, said it had reached a size where it needed outside investment to fund its future development. "I know Omnicom and it is the only group I ever wanted us to join. We have a shared interest in the same things," he said.