Shanklin won the Grand Slam with Wales on two occasions and his last appearance for his country came in the against New Zealand last November.
"My knee isn't too bad, it's ok to walk and cycle and to do everyday things, but with regards to playing rugby: accelerating, changing direction, weight lifting, it's not too good," said Shanklin.
"I had an operation in January to see if we could fix the problem and it hasn't worked."
"It's bone on bone on my knee which has restricted me from doing a lot of things and I'm going to have to retire from the game."
"The advice from Rhys Williams the surgeon was to retire. Rhys knows the inside of my knee pretty well having done three or four operations on it."
"It was down to me to decide if I can continue or not, but I can't go on, as it's too sore and it's a pretty black and white situation. I can't fully function with it and I need to start taking account of my body and start listening to it."
"I'd like to thank not only Rhys for all his work, but all the Blues medical team who have been excellent in all they have done to keep me on the field for this long."
"I've had a history with a bad knee, but since 2005 it hasn't really troubled me. I've managed to carry on and continue until I injured it again January. I was aware at the time that it could end my career."
"A lot of the decision on whether I would retire was down to how my knee would recover and how I felt."
"I've now had time to take it on board, so it isn't a shock, and I've come to terms that I'll never play again."