World Rugby, the Rugby Football Union and Welsh Rugby Union have confirmed they have received a solicitor's letter which alleges a failure to protect players from the risks caused by concussions.
The letter of claim, setting out their intention to sue, has been sent to the sport's governing bodies with a total of nine players involved, including England's Steve Thompson and Michael Lipman and former Wales flanker Alix Popham.
"World Rugby, the Rugby Football Union and Welsh Rugby Union can confirm they have received a letter of claim from solicitors representing certain players and will now take time to consider its contents," the governing bodies said in a joint statement.
"We have been deeply saddened to hear the brave personal accounts from former players. Rugby is a contact sport and while there is an element of risk to playing any sport, rugby takes player welfare extremely seriously and it continues to be our number one priority.
"As a result of scientific knowledge improving, rugby has developed its approach to concussion surveillance, education, management and prevention across the whole game. We have implemented coach, referee and player education and best practice protocols across the game and rugby's approach to head injury assessments and concussion protocols has been recognised and led to many other team sports adopting our guidance.
"We will continue to use medical evidence and research to keep evolving our approach. As with any potential legal proceedings, it would be inappropriate to comment on the specifics of the letter."
World Rugby chairman Bill Beaumont released a statement that didn't address the legal letter but said player welfare was his top priority.
"My thoughts are with them and anyone in the rugby family who is struggling," Beaumont said in an open letter on Thursday.
"I want to reassure every member of the rugby family that player welfare is -- and always has been -- our number one priority at all levels of the game.
"As a player who retired on medical advice in the early 1980s, I care deeply about the welfare of all players. They are the heartbeat of our sport and we work tirelessly to protect them.
"Both the game and the level of player care and welfare have evolved significantly since my playing days and, while we are never complacent, I have witnessed first-hand how advances in science and medicine have informed our protection of players.
"I will continue to lead on safety and do all I can to maintain the confidence and wellbeing of those who love playing the game."
RFU chief executive Bill Sweeney had previously pledged that his organisation would remain "open and transparent" in reacting to the specific situation, but also in continuing its work to minimise concussions and improve player safety in rugby.
"It's a very serious matter, a very serious moment for us; we all love this game," Sweeney added. "First and foremost in these legalistic times this is very much a human story.
"And we recognise what's happening here, we recognise the difficulties the families are going through, and then bringing these stories into the public ... to improve the game and make it better and safer for future generations.
"We would certainly share that and take it to heart. And it's impossible not to be moved by it. It's not a time to hide, not a time to go missing; it's really a time to be open and transparent."