More than 700 children are born with genetic disabilities in the UK as a result of the cousin marriage system.
The research showed that the problem is worst among children born in Britain's Pakistani community, where more than half of marriages are between first cousins.
Children born from such unions suffer genetic disorders, and the medical risks include higher rates of infant mortality, birth defects, learning difficulties, blindness, hearing problems and metabolic disorders.
As adults these children are at an increased risk of miscarriage or infertility, while a third of children affected die before their fifth birthday.
The investigation by Channel 4's Dispatches programme found that even though more than 70 British studies have proved the risks, many people are still denying the dangers, and first cousin marriages continue to rise.
Ann Cryer, the former Labour MP for Keighley, suffered abuse for trying to highlight the problems.
"It's a public health issue and we deal with public health issues by raising awareness, by talking about subjects such as obesity, such as drug addiction, such as alcohol," the Telegraph quoted her as saying.
"But for some reason we're told that we mustn't talk about cousin marriages because this is a sensitive issue.
"I think it's absurd, we have to talk about it in order to find solutions," she added.