In a meeting with agency management last week, Barker decided not to renew his contract, which is due to expire at the beginning of next year.
After the management talks Barker took the decision to leave the agency at the end of the month to seek a fresh challenge.
Barker said: "We could have waited until March to have those discussions, but there didn't seem much point to that. It seemed unfair to ask people to shift round for my lack of enthusiasm."
Chris Cowpe, BMP's joint chief executive, added: "Larry has basically found his role is not fulfilling what he wants to do. We have been talking about what he and we want to do, and it's better to do it and get on with it."
Barker moved off BMP's creative floor in May into a broader management role. At the same time, the joint creative directors, Ewan Paterson and Jeremy Craigen, stepped up to take sole charge of the creative helm.
"Larry left us the legacy of Ewan and Jeremy and the department we've got," Cowpe said. "We're very grateful."
Barker will be on gardening leave until the end of the year, but after that time will be looking to get back to his creative writing skills in a broader role, which may include consultancy or project work. "I'd certainly like to be using my craft a bit more," he said.
Barker joined BMP as the creative director in January 1998 when his predecessor, Tony Cox, left to join Abbott Mead Vickers BBDO. Before that he had been WCRS' joint creative director alongside Rooney Carruthers, where, among acclaimed work for clients including First Direct, their biggest highlight was the launch of Orange. Before that they had been at Bartle Bogle Hegarty, where they created work for Haagen-Dazs and the award-winning "swimmer" ad for Levi Strauss.
Barker started his career at BMP nearly 22 years ago, where he first carved a reputation for himself through campaigns for Foster's, Alliance & Leicester and Channel 4. He said: "I won't cut off all connections with this business. I never would, because I've spent 20 years doing it. But I'm not sure I'd want to be tied up with the fortunes of a company any more." Over the past year, BMP's billings have fallen by 36.7 per cent.
Craigen has been at BMP for 12 years since leaving Bates Dorland, with Paterson joining from Young & Rubicam in 1995. They were both promoted to deputy creative directors last October.