Johno Hangs Up His Boots
The Forest striker, now 30, underwent surgery for the disc problem at the end of last year but has failed to regain full fitness and has been advised to hang up his boots.
Johnson said: 'I worked throughout the summer trying to get myself to a good level of fitness. When we went to Italy on the pre-season tour, I joined in with the rest of the lads for the first training session and was in some discomfort.
'I trained again the next day and was in so much agony that I started to doubt in my own mind whether it would ever clear up.
'I had a long chat with the manager about it and then spoke to several doctors, and on their advice I decided the time was right to announce my retirement.
'The doctors all agreed it was in my best interests to call it a day.'
The frontman scored 50 times in 167 appearances.