Manchester Royal Hospital tested hundreds of patients
As many as 40% of people who have been given strong drugs for epilepsy have got heart problems which could be treated, it claims.
The researchers say sufferers are not having epileptic fits, but that their bodies are twitching after they pass-out due to a sudden drop in blood pressure.
Epilepsy is caused by temporary electrical changes in the brain.
Heart rate
The study has been conducted by Dr Adam Fitzpatrick, consultant cardiologist at Manchester Royal Infirmary, and Dr Paul Cooper, consultant neurologist at Salford's Hope Hospital.
Dr Fitzpatrick told BBC News Online: "These people are suffering from a cardiovascular condition which makes them faint because of a sudden drop in blood pressure.
"We don't know exactly how it is triggered but it is accompanied by a slowing heart rate.
"Epileptics have a stiffening of the limbs, followed by a rhythmic jerking.
"To the untutored eye it can appear that the people who are fainting are also having an epileptic fit.
"If they collapse in the street a witness may tell an ambulanceman they have seen someone having a fit and that misdiagnosis is passed on to casualty and then on to their GP."
Dr Fitzpatrick said being misdiagnosed as epileptic could have three main effects on patients' lives.
'Stigma attached'
He told BBC News Online: "There is still a stigma attached to epilepsy in today's society.
"It can also have an effect on whether they are allowed to drive or not.
"And perhaps the most important issue is that of child-bearing.
"The drugs used to suppress epilepsy can damage the developing foetus.
"I know of one instance when a woman was treated for epilepsy and had children born with birth defects.
"She was then told she didn't have epilepsy at all."
He said correct diagnoses meant patients could be fitted with pacemakers and given effective drugs to counteract fainting.
Chest implant
The Manchester research has studied 900 patients over the past four years, many referred to them by GPs, physicians, and neurologists.
Some of them have been fitted with a machine called a Reveal - implanted into the chest near the collar bone - which monitors a patient's electro-cardiogram for up to two years.
After a blackout the patient can press a button which freezes the data records of their heart activity prior to the attack. This can then be downloaded.
The researchers hope to establish a clinical network to gather and distribute information about the fainting condition to patients, GPs and other medical professionals.