Doncaster Knights prop forward Ian Williams has died after collapsing in training on Tuesday morning.
The 27-year-old Oxford University graduate joined the Knights last summer from neighbours Rotherham Titans and made six appearances.
A statement from Doncaster RFC read: "It is with great sadness that Doncaster RFC announces the passing of prop forward Ian Williams.
"Ian collapsed at training at Castle Park this morning and, despite the best efforts of the emergency response team, could not be resuscitated.
"We extend our deepest sympathies to Ian’s mother Pippa, father Phillip and and sister Helen and are working with a counselling team in the best interest of Ian’s teammates."
Tributes for Swansea-born Williams came in quickly from across the Doncaster sporting community and beyond.
Club Doncaster chief executive Gavin Baldwin said: “Everyone at Club Doncaster extends our best wishes to the family of Ian, as well as everyone at the Knights.
“Ian was a prop with plenty of his career ahead of him, this is a sad loss for Doncaster’s sporting community.”
Dons chief executive Carl Hall said: “The Dons family stands alongside the Knights at this sad time.
“We have a strong working relationship with the Knights, and have been training at Castle Park this year. Our thoughts are with them and Ian’s family.”
His previous club Rotherham offered their condolences, writing: "Ian was a popular member of the squad here at Rotherham and although he was only here for a year before moving on, he made a big impression among everyone.
"It is a real tragedy and a shock and we send our condolences to his family and friends and to everyone at Castle Park."
The Rugby Football Union also issued a statement on his passing.
It read: "On behalf of everyone at the Rugby Football Union and the Greene King IPA Championship we extend our heartfelt sympathies to Ian’s family, friends and team mates.
"It will be a difficult time and they can be assured of our support."
His former club Richmond tweeted: "All at the club are devastated to hear the news of the passing of former Richmond youth and seniors player Ian Williams. Our thoughts our with his family, friends and all at @DoncasterKnight."
The prop made 29 appearances for Swansea RFC and is also understood to have played for Bridgend Ravens, Glamorgan Wanderers and Mumbles RFC.
An accomplished clarinet player, Williams studied History at Swansea University before taking a Historical Studies course at Oxford, where he then earned a MSt in Literature and Arts.