The small study was the first of its kind, involving postmortems on six men who died with dementia after long careers playing football. All were skilled headers of the ball. Scientists have found signs of brain damage that could cause dementia in a handful of former football players, fueling worries about the danger of frequent knocks from heading the ball or colliding with others on the field.

But experts said more research was needed to prove any definitive link between heading a football and developing dementia, and they added that the risk was likely to be minimal for occasional players.

It suggests that some professional football players might risk the same long-term cognitive problems suffered by boxers and some American football players.

“We’ve demonstrated that the same type of pathology that occurs in ex-boxers can also occur in some ex-footballers who have dementia, but I’d emphasize this is a very small number of players,” said co-lead researcher Huw Morris of London’s UCL Institute of Neurology.

The scientists found evidence of chronic traumatic encephalitic (CTE), a potential cause of dementia, in four of the six brains. All six also had signs of Alzheimer’s disease.

“The average playing career of these players was 26 years, which is thousands of hours of game playing, thousands of hours of practice and thousands of headers. I think the risk is extremely low from playing recreational football.”

Unlike boxing or American football, blows to the head in football are generally lower impact and players are less likely to experience concussion. But there may still be cumulative damage from sub-concussive impacts, experts believe.

Britain’s Football Association said more work was needed into whether degenerative brain disease was more common in ex-footballers, adding it planned to jointly fund research with the Professional Footballers’ Association.