Wales lose star to Epilepsy
Thomas, who helped guide the Warriors back to the Premiership last season is thought to have sustained multiple head traumas over his career, and this has led to a degree of brain injury.
The Warriors loose forward has now been forced to hang up his boots following medical advice.
Warriors 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."
During a successful international career, amassing 67 caps for Wales, Thomas played a starring role at the 2003 and 2007 World Cups.
He was named in eight consecutive Six Nations squads between 2004 and 2011, featuring in two Grand Slam winning teams.
At club level, Thomas enjoyed 10 years at Welsh region Ospreys, where he made a club-record equalling 188 appearances, including four league titles and an Anglo-Welsh cup.
He then moved to Sixways in 2013, making 44 appearances for Warriors in two seasons.
Director of Rugby Dean Ryan said: "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."
