The actor, who has been a part of the Street for 30 years, will depart as Mike Baldwin loses his battle against Alzheimer's disease.
A spokesman confirmed to the News of the World, "His storyline has some way to run but he will be leaving in 2006 after completing 30 years in the show. It was his decision to retire."
Johnny's current two-year contract expires next autumn, and he has already turned down another £200,000-a-year deal.
Last year, Briggs admitted that he struggled with the show's busy filming schedule, commenting, "When I started we were filming two episodes a week and now we're doing five."
He added, "I'd like to try other acting work like movies."