The ex-Wales U20s ace suffered left-sided peripheral nerve damage as he made a try-saving tackle in February 2018.
Williams has undertaken extensive rehabilitation at Cardiff Blues with regular expert assessments. However, the 22-year-old has now been advised that this is a career-ending injury at this time.
He said: “I’m obviously really disappointed to be given the news that I will be unable to return to rugby. I have done everything I can to get back but unfortunately, that’s the specialist’s advice at this time.
“I’m very grateful for all the support I have had from everyone at Cardiff Blues during the last two years, but particularly the physios and medical department, Phil Davies from the WRPA and my team-mates who have been superb.”
Williams, who began playing rugby at Caernarfon, joined the Blues academy in 2016 and went on to make 28 senior appearances for the region.
He also starred in the 2016 U20s Grand Slam-winning Six Nations campaign with Wales and was named in the senior squad the following year after impressing on the regional stage.
Blues chief executive Richard Holland added: “It’s with great disappointment that Rhun has been advised to stop playing.
“We all saw the ability and potential that Rhun possessed, but all the more impressive has been the way he has dealt with this injury and how hard he has worked to give himself every chance of returning.
“Unfortunately that wasn’t to be but he remains part of our family and we will look to continue supporting him as he adapts to life outside of rugby.
“My thanks also go to the staff at Cardiff Blues, particularly within the medical and strength and conditioning departments, for the invaluable effort and support they have given to Rhun. We wish him all the very best for the future.”
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