One serious head injury on the sports field could be enough to trigger dementia in later life, a major study suggests.

The research by Imperial College London, which highlights the risk of single head injuries, comes amid growing concern about the potential damage which could be caused by football and contact rugby.

The study scanned 21 patients who had suffered severe head injuries at least 18 years later, and compared them with 11 individuals who had not.

For the first time, scans of living patients found protein “tangles” associated with dementia in the brains of patients who have suffered a single head injury.

Tau tangles are found in Alzheimer’s disease and other forms of dementia, and associated with progressive nerve damage.

Scientists have known for some time that repeated head injury – such as those sustained in sports such as boxing, football and rugby - can lead to neurodegeneration and dementia in later life, with particularly strong links to a type of brain condition called chronic traumatic encephalopathy (CTE).