Cull joined MSLGroup London as MD for corporate affairs in June after his previous job at Vodafone was scrapped last year.
Cull was recruited by former CEO of MSLGroup London Jeremy Sice, who stepped down from the role in September.
Sice has been replaced by joint CEOs James Parsons and Jason Frank. Parsons and Frank are in the process of rebranding MSLGroup and focusing on providing clients with a new digital and social offering.
Parsons said: "We are making changes here that mean we are not going to need to replace Christian for the time being."
He added: "Christian was a big hitter in the corporate world but there are many challenges coming from the client side. Senior hires are really challenging and we are sad we didn't get it to work. It was a mutual decision, if it's not working in the early days what we tend to do is reach an amicable arrangement."
Cull told PRWeek that he does not have another job yet but is keeping his options open. At Vodafone UK he led PR around the launch of 4G and the integration of Cable&Wireless Worldwide but found himself out of a job after changes made by the company’s new CEO Jeroen Hoencamp.
Before this Cull held a number of high profile positions at major corporates across broadcasting, travel and telecoms, including comms director of TUI UK & Ireland and director of customer comms at BSkyB.