Hull midfielder Ryan Mason has been forced to retire due to a head injury suffered in January 2017.
Mason suffered a fractured skull in a collision with Chelsea's Gary Cahill during a game at Stamford Bridge last year and has not been allowed to play since.
The former England international had been working towards a comeback but will retire on the advice of medical professionals.
Mason made a total of 69 Premier League appearances after breaking into the first team at Tottenham in 2014, before signing for Hull two years later.
In a statement, the 26-year-old said: "I can confirm that following specialist medical advice I am having to retire from playing professional football.
"I have worked tirelessly in order to be able to return to the pitch. Unfortunately, having taken expert medical advice I now have no option but to retire due to the risks involved given the nature of my injury.
Everybody at Chelsea Football Club sends our best wishes to Ryan Mason following his retirement. Good luck in whatever you choose to do next, Ryan.— Chelsea FC (@ChelseaFC) February 13, 2018
"I am and always will be eternally grateful for the incredible network of people around me who have helped me to recover from what was a life-threatening injury in January 2017.
"I am so proud to have come through the ranks to play for my club, the club I love. To have had the honour of captaining the team fills me with immense pride. Thank you to all of my teammates over the years, there are too many to mention, it's been a privilege to play alongside you all.
"Finally, representing my country is an honour that nobody can ever take away from me and something that I am so proud to have achieved.
"I'm retiring from the game with my head held high having done everything in my power over these past 13 months to have given myself every opportunity to return to the pitch."
A statement on the Hull's official club website read: "Ryan would like to put on record his thanks to all at the club who have aided his recovery to this point and he is indebted to them for their support and compassion over the past 12 months."