Cardiff Blues have revealed Wales centre Owen Williams has suffered a "significant injury" to his cervical vertebrae and spinal cord.
Williams is recovering in hospital in Singapore after undergoing an operation to stabilise a fracture, Cardiff said in a statement.
The 22-year-old was injured while playing for the region at the World Club 10s in Singapore.
The Blues say arrangements are being made for his "safe transfer home".
Williams's family are at his bedside, along with senior Blues officials and medical staff.
Players from around the world have taken to Twitter to support four-times capped Williams.
Wales captain and Blues club-mate Sam Warburton tweeted the hashtag "#StayStrongForOws" while Toulon, British and Irish Lions and Wales full-back Leigh Halfpenny wrote: "Thoughts are with you and your family."
Australia flanker David Pocock wrote: "Owen, sending some love and well wishes from Canberra. We're all thinking of you and hoping for a speedy recovery."
Williams's brother, Gavin, tweeted: "So emotional reading all the support. It's overwhelming. In the hospital next to my brother. He's a warrior."
The Welsh Rugby Union offered its support and best wishes to Williams, and said it would "continue to liaise closely" with the Blues.
Blues said in a statement: "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."
Williams came through the Blues academy system after being spotted playing for Neath College.
He signed for the senior side in 2010, but only made nine Blues appearances last season because of a hamstring problem.
The former Aberdare RFC player was capped by Wales at under-16 and under-20 level and made his senior debut against Japan in June 2013.