The incurable brain illness Huntington's Disease (HD) is at least twice as common as previously thought, medical experts said today.
New statistics on the prevalence of the condition in the UK suggest that the numbers affected by it have been misjudged.
Sir Michael Rawlins, chairman of the National Institute for Health and Clinical Excellence (Nice), believes that official figures for those who are symptomatic of having HD are wrong by a factor of at least two.
Writing in medical journal The Lancet, he argued that the stigma associated with the disease has led to a serious under-estimation of its prevalence.
He said: "Stigma (of having HD) has had a deleterious effect on studies that have sought to investigate its epidemiology and... the true prevalence is unquestionably greater than this."
HD is a hereditary disorder which affects muscle co-ordination and cognitive functions.
It usually first shows itself in middle age and in time leaves its sufferers needing full-time nursing care before they die.
Every child of someone with HD has a 50% chance of inheriting the disease.
About 6,000 people are believed to have the disease in the UK.
But Professor Rawlins believes the true number is somewhat higher.
Speaking on the BBC Radio 4 Today programme this morning, he cited Ian McEwan's 2005 novel Saturday as a cultural example of how those with HD are stigmatised.
He said: "There has been a terrible stigma associated with Huntington's Disease over the years. People with Huntington's Disease were reputedly 'bad' people.
"They are still stigmatised. Ian McEwan, in his novel Saturday - the villain was villainous because he had Huntington's Disease."
The Huntington's Disease Association welcomed the recognition of the condition's prevalence.
Cath Stanley, head of care services at the charity, said: "This is of huge importance for the people we look after because HD is always thought of as a very rare illness and there's little support for people.
"If incidents are higher than previously thought it has implications for the provision of care for people with HD."
The charity is calling for more support for people with the disease and for more funding for their work.
Ms Stanley also agreed that there was a stigma attached to the disorder.
She said: "I think it's because of the history of hereditary disorders and mental health problems, people are embarrassed to say they've got the disease.
"It's also because the support is so limited and there's a lack of knowledge of the illness."