Scientists believe they've found the culprit behind nearly all neurological diseases, in a breakthrough that could offer hope to millions.

Dementia, Parkinson's and many other brain disorders are caused by key cells called neurons dying over time. 

Researchers have now found other brain cells – known as astrocytes – play a 'critical role' in their death.

The star-shaped cells usually help clear away toxic particles that build up in the brain naturally or after head trauma, and are supposed to nourish neurons.

But laboratory tests on mice show astrocytes also release toxic fatty acids to kill off damaged neurons, confirming a suspicion many neurologists have had for years.

Lead researcher Professor Shane Liddelow, of New York University, said: 'Our findings show the toxic fatty acids produced by astrocytes play a critical role in brain cell death.'

He added that the results 'provide a promising new target for treating, and perhaps even preventing, many neurodegenerative diseases'.

Dementia is the leading cause of death in UK and the developed world, with modern habits and increasing life expectancy to blame. It kills just over 66,000 Britons every year — the equivalent of one every 10 minutes.

Neurologists are racing to develop treatments but still don't fully understand some key aspects of the disease, such as what triggers the disease in some cases.

Astrocytes — which take the shape of a star — were known to keep neurons in the brain nourished and healthy and regulate bloodflow.

But the NYU scientists say they are also responsible for killing decaying neurons.

After coming up with the theory that two fatty acids were to blame, the NYU researchers tested it in mice with brain injuries.