Wales international forward Jonathan Thomas has been forced to retire from rugby at the age of 32.
In a statement, his club Worcester said that Thomas was diagnosed with epilepsy last year "which is thought to be from multiple head traumas and has led to a degree of brain injury."
Thomas, who won 67 Wales caps between 2003 and 2011 and played in two World Cups, has now stepped down on medical advice.
News of his retirement comes as the World Cup prepares to kick off later this week, and delivers another stark reminder of the debate around head impacts and concussion which has dominated rugby union's agenda in recent months.
Worcester high performance director Nick Johnston said: "We've sought the best medical opinion in the country in order to try and enable Jonathan to prolong his career.
"But unfortunately, on the advice of consultants, he has been advised to retire from professional rugby."
Thomas also featured in two Six Nations Grand Slam campaigns with Wales and spent 10 years with the Ospreys before moving to Worcester in 2013.
Warriors rugby director Dean Ryan added: "It's with great sadness that after all his efforts Jonathan has had to concede his fight to continue playing rugby.
"This is a great opportunity to celebrate all of his success as a player, but also to go on record in how grateful I am in the courageous decision he took to come and join me here at Warriors.
"Some of our greatest challenges have come during these first two years to ensure we redirect this club to a more successful future, and Jonathan's actions have been fundamental to this.
"I wish him and his family success in their next steps in life."