As part of the agreement struck by Viacom to bring the contract to an end, a number of Outside Organisation staff will become Channel 5 employees.
However, the agency is retaining work related to the promotion of Channel 5’s key ‘Big Brother’ programming, which has in the past seen it work closely with former Channel 5 owner Richard Desmond’s other media assets such as OK! magazine and the Daily Star.
Desmond recruited Outside after he bought Channel 5 in 2010 and the in-house communications team took voluntary redundancy.
A spokesman for Viacom said: "The Outside Organisation has done a fantastic job on our behalf but it will be easier to align Channel 5’s communications strategy with Viacom’s by taking more direct control of this important function.
"We’re parting with Outside on amicable terms and they will continue to publicise Big Brother. We thank them for their professionalism and look forward to working with them on new projects in the future."
The UK comms operation at Viacom, which is US-headquartered and also owns MTV, is headed by former Channel 4 comms chief Matt Baker, who was recruited as vice-president of corporate comms for Viacom International Media Networks in early 2013.
The owner of Outside, Alan Edwards, said: "It is always sad to part company with any client but it's always a likely outcome when a business is sold.
"I am very proud of the job Outside did for C5 from handling the publicity around Richard Desmond's purchase of the channel right the way through to its recent sale to Viacom.
"There is no doubt that C5's standing in the media generally has improved massively during the time that we handled their publicity and it was sold very profitably which is a credit to all the teams involved!"
Desmond paid German broadcaster RTL £104m for the channel and sold it to Viacom in September for £450m.
Edwards added: "We will continue to handle the PR for Big Brother as we have done since the very beginning and obviously wish the channel continued success going forward."