London sports law firm Rylands Garth said it would issue proceedings in court on Tuesday on behalf of 100 rugby league players, 40 rugby union players and 15 football players, taking the total number of claimants to 380.
The players allege that the sports' governing bodies failed to protect them from concussion and non-concussion injuries that caused various disorders including early onset dementia, chronic traumatic encephalopathy, epilepsy, Parkinson's disease and motor neurone disease.
Reuters has contacted World Rugby, RFU, WRU, RFL and FIFA for comment.
In a joint statement last year, World Rugby, the RFU and WRU said they "never stand still" when it came to player welfare.
"Our strategies to prevent, identify and manage head injuries are driven by a passion to safeguard our players and founded on the latest science, evidence and independent expert guidance," they said.
World Rugby has also recommended the tackle height be lowered in the amateur game and pointed to studies in France and South Africa that showed positive advancements in terms of player safety and overall game experience.
Rylands Garth represents over 250 rugby union players with brain damage, including England World Cup winner Steve Thompson and former Wales captain Ryan Jones, in a claim against World Rugby and the governing bodies of England and Wales.
The firm also represents 100 rugby league players as part of a separate but similar potential claim against England's RFL.
Former British and Irish Lion Dafydd James, who joined the claim on Tuesday, said his early onset dementia diagnosis could explain his mental health struggles.
"In a way it probably highlights that I've got a little bit of an answer about why I feel the way I do," the former Wales international told the BBC.
"I think there's a duty of care on both sides to make it safer so there's longevity and the game can move forward."
