Some of the Yorkshire-based clients, including Derbyshire Building Society and Cavendish Building Society, have moved with Diamond and Moore to the new agency, with the remaining PHD business transferring to London and Manchester.
Diamond said the agency, which has been operating since the beginning of June, was currently pitching for new business and planned to recruit for two new roles.
He said: "We wanted to do something different and the move has been entirely amicable. It's an exciting time for us and we'll continue to work closely with PHD in the future."
Morag Blazey, managing director of PHD, added: "We believe this to be a great move for Andrew, Bridget and their founding clients. While we are very sorry to see them go, we wish them every success and are thrilled to be working together on some elements of the business."