Mears "was informed that there is a significant potential risk of developing further complications if he continues to perform the prolonged strenuous exercises required to be an elite rugby player."
The 33-year-old said in a statement: "This is not how I saw it ending but I feel enormously proud to have played for such a club so many times. This will be a tough period, but the memories I have and the friends I have made over the years will be of huge support.
"Thank you to the club and to the supporters, I've loved every minute."
Head coach Gary Gold added: "The only thing that matters is that he is healthy, and if that means he stops playing, then that's the way it has to be.
"He will be missed, but we will always welcome him back with open arms."
Mears made his debut for England in 2005 against Samoa and went on to earn 42 caps for his country. He was a member of the England squad at both the 2007 and 2011 World Cups.
He was also a member of the British and Irish Lions squad that toured South Africa in 2009 and started the first Test.
He retired from international rugby in 2012 when new coach Stuart Lancaster said he was no longer in his plans.