Lenny Woodard believes rugby caused his dementia.
On Monday he, ex captain Ryan Jones, and 180 others intend to issue formal proceedings against World Rugby, the Welsh Rugby Union (WRU) and the Rugby Football Union (RFU).
World Rugby, the WRU and RFU said they could not comment on any case as no legal proceedings had been received.
However, in a joint statement, they said they cared deeply about all players and had strategies to prevent, identify and manage head injuries.
London-based law firm Rylands Legal is representing the claimants, arguing rugby's governing bodies failed to take reasonable action to protect players from permanent injury caused by "repetitive concussive and sub-concussive blows".
The firm said it was the biggest class action of its kind outside the United States, where the NFL has paid more than £664m to date to former players who developed dementia or concussion-related problems.
"This claim isn't just about financial compensation," Rylands said in a statement. "It is also about making the game safer and ensuring current and former players get tested so that if they are suffering a brain injury they can get the clinical help they need."
Former dual-code pro Mr Woodard played rugby union for Pontypool, Ebbw Vale and Pontypridd, and although not capped for Wales he played on their 1998 tour to South Africa and won five caps for the Wales rugby league team.
He said others have not yet spoken about their condition as they cannot face telling their families.
The dad-of-five said he first suffered a concussion aged 11.
The 45-year-old said: "The stark reality of being diagnosed with early onset dementia in your 40s, with young children, means in five years, 10 years, 15 years time we might not know who they are."
He recounted discussing the matter with his partner as his children played in the next room.
He said: "My partner was explaining, 'I want the kids to see you as much as possible, as often as possible and do as many memorable things as possible because there is a chance in 10 years, 15 years time you're not even going to know who they are.'
His nine-year-old son, Leo, overheard the conversation.
"He burst into tears, ran into the room, hugged me and would not let go of me for a few minutes," he said.
Pontypridd-born Mr Woodard, who was diagnosed last year, said he was aware of others in the game in a similar situation to himself.
He said: "The reason they haven't gone public with it is because they're struggling to tell their children, wives, family."
When speaking to other players he has assured them they are not alone.
And he warned: "There's going to be more."It was hard to tell people who had played at the sport's top level what lay ahead.
"Unfortunately the game is going to take away their enjoyment of their later life," he said.
Mr Woodard said he had gone from having "perfect" recollection to regularly repeating himself.
"My memory, my thought processes, are declining gradually and I can see that," he said.
"I was diagnosed a year ago and I can see it has got worse over the last year.
"I'm hoping to slow that acceleration as much as possible. I often forget journeys at crucial stages within it, repeat myself in conversations, repeat myself half-an-hour later, an hour later, forget conversations, things I've done in the past completely now.
"Whereas five years, 10 years ago I had what I would class as a perfect recollection of things."
Progressive Rugby, a non-profit group demanding better protection for players, is not involved in the court action, but said it was in the process of "finalising a comprehensive list of player welfare critical requirements" to submit to World Rugby.
"We believe delay is no longer an option and that radical action must be taken as a matter of urgency to ensure rugby union's reputation isn't damaged beyond repair," they said.