Saracens lock forward Alistair Hargreaves has retired from rugby with immediate effect following a concussion injury.
Hargreaves made 79 appearances for Saracens over four years, leading the club to the Premiership title in 2015.
The 30-year-old said: "It's been a very tough decision to make. I've suffered a number of concussions in the last two seasons and after seeking medical advice I've come to the conclusion that the risk is outweighing the reward.
"I have a young family, I have to make decisions that are right by them and although this was a hard decision for me personally it's one that I feel is the right one to make.
"I leave as a Saracens man to the core. My time in South Africa was fantastic, it was great to play for the Sharks and be capped by the Springboks but this time at Saracens has been my highlight. I've grown as a rugby player but more importantly, I've grown as a human being."
Saracens director of rugby Mark McCall added: "The welfare of our players is of paramount importance - we are fully supportive of Alistair's decision.
"He has played a key role in our recent success with his character, hard-work and leadership coming to the fore over the last four seasons. We wish Alistair, his wife Carmen and their young son the very best for the future."
Earlier this year, Saracens appointed centre Brad Barritt as captain he led the side to the Premiership and European Champions Cup titles while Hargreaves was sidelined through injury.
Together with former team-mate Chris Wyles, Hargreaves has launched a brewery and the pair are considering opening pubs and creating more beers.