Cardiff Blues and Wales centre Owen Williams "suffered a significant injury to his cervical vertebrae and spinal cord" in Singapore.
Williams, 22, was hurt while playing for the Blues in the inaugural World Club 10s tournament on Sunday.
The Welshman was injured in a third and fourth place play-off game between the Blues and Asia Pacific Dragons, which took place at the country's National Stadium.
Williams' family are by his bedside and arrangements are being put in place for his transfer back to the United Kingdom.
In a statement the club said: "Cardiff Blues have today (Wednesday, June 25) confirmed that Owen Williams has suffered a significant injury to his cervical vertebrae and spinal cord.
"He underwent an operation on Monday to stabilise a fracture and is currently recovering in hospital in Singapore.
"The family of Owen Williams are now at his bedside along with senior Blues management and medical staff.
"Arrangements are currently being made to co-ordinate his safe transfer home at an appropriate time.
"Cardiff Blues and the Williams family would like to thank the many hundreds of well-wishers who have offered messages of support.
"Cardiff Blues will be making no further comment, but will continue to share further updates when they are available.
"We would also ask that the privacy of Owen Williams and his family is respected at this time."
Wales international team-mates Alex Cuthbert, Jamie Roberts, George North, Dan Lydiate, Ken Owens and Josh Turnbull have offered Williams support on social media in addition to Australia back-row star David Pocock.
Gloucester Rugby Club, Leinster, the Scarlets and Newport Gwent Dragons have also expressed support for Williams, as well as Cardiff City Football Club.
Williams has won four Wales caps, making his debut during last year's tour to Japan and then scoring a try in a 17-7 Millennium Stadium victory over Tonga seven months ago.
He has also represented Wales at U20 level and on the international world sevens circuit, while he joined the Blues' senior set-up after progressing through the region's academy system.
His former club Aberdare, meanwhile, have postponed an upcoming presentation event that was planned to recognise ex-Aberdare players who achieved international honours.