"I have chosen to retire immediately through strong medical advice," Harrison said.
"The advice I have been given stipulates that if I continued to injure the nerve in my neck I'm at risk of losing all feeling in my arm and, at this stage of my career, I would be foolish to jeopardise my long term health," he said. "Nor do I feel I could have contributed to the team as well as I would want to with that in the back of my mind.
"I'd like to thank everyone who has been a part of my career in some form or another; they know who they are. I have been blessed to play the game I love for such a long period of time."
Titans coach John Cartwright hailed Harrison's professionalism and said it was a disappointing end to a great career.
"You don't get to pick how you're going to finish," he said.
"He deserved to go out with a premiership if that was possible.
"Having said that, if he looks back on nearly 15 seasons in the NRL, the highlights will far supersede any memory of how he finished."
- AAP