A blood test the makers claim can detect Alzheimer's and Parkinson's up to six years before they take hold is to be launched this summer.
The test will allow doctors to alert those at risk of developing the conditions, and advise them of lifestyle changes such as improvements to their diet or more exercise.
It could also allow earlier treatment with drugs which slow the progress of the diseases.
But the breakthrough has raised fears that insurance companies could force people to undergo the check - and raise premiums for those deemed at risk.
Said by its creators to be 90 per cent accurate, the test measures the blood for proteins which can indicate the risk of brain disease, according to a report in Chemistry & Industry magazine.
U.S.-based manufacturer Power3 Medical Products plans to market the test in the U.S. and Greece later this year.
Chief executive Steve Rash said: "There is currently no diagnostic test for any neurodegenerative disease on the market. Diagnoses are currently based solely on a clinical diagnosis of symptoms.
"We are on the brink of offering this blood serum-based test that will meet a tremendous need for early detection methods of neurodegenerative disease."
The company says that by measuring a collection of 59 protein "biomarkers", the Nuro Pro test can even distinguish between Parkinson's, Alzheimer's and motor neurone disease.
Patients are provided with a probability score based on the test result, which compares their blood sample with a biological and statistical model developed by the company.
Scientists are in the final stages of testing with a study of 300 patients - 100 of whom have Alzheimer's, 100 Parkinson's sufferers and 100 healthy patients.
At present, doctors rely on interpreting physical signs when a patient complains of symptoms, in some cases backed up by brain scans.
But genetic analysis has made it possible to detect proteins in the blood linked to the diseases.
Power3 has used this technique to develop a blood serum test which involves a blood sample taken from a vein.
Kieran Breen, director of research at the Parkinson's Disease Society, welcomed the test but warned the research was in its early stages.
He said: "While the test seems promising, larger studies need to be conducted before it can be confirmed as being helpful in making a diagnosis."
Dr Susan Sorensen, head of research at the Alzheimer's Society said: "There are 700,000 people living with dementia in the UK, 62 per cent have Alzheimer's disease and this will rise to more than a million in less than 20 years.
"An effective blood test would present those diagnosed and their families with an opportunity to prepare for the impact of this devastating illness and make crucial decisions about their future."
Last year the drugs rationing watchdog, the National Institute for health and Clinical Excellence, ruled that the drug Aricept should only be given in the later stages of Alzheimer's.
Its decision was later upheld by the High Court. FISH OIL BRAIN BOOST
A major breakthrough in the battle against Alzheimer's is being claimed by British scientists who believe it can be fought with omega-3 oils.
They are found in nuts, seeds and oily fish like mackerel and salmon.
Researchers at Aberdeen University found that some older people whose diets are high in omega-3 oils do better in mental tests than those without the oils in their diets.
The research also found a genetic link that explains why some fish oil studies have been inconclusive in the past.
Scientists studied 120 over-64s who had sat intelligence tests in Scottish schools in 1947.
They were tested again at the ages of 64, 66 and 68, with blood samples taken to identify those who possessed a gene that made them more likely to develop Alzheimer's.
For those aged 68 without the crucial ApoE e4 gene there was a clear link between better results and the presence of omega-3s in their blood.
However, in those with the gene, which is linked to contraction of the disease earlier than usual, the oils made no difference to their scores.