Tens of thousands of Asian people living in the UK risk dying from liver disease, researchers fear.
A blood test can detect hepatitis C infection
Their work suggests most first generation Asians infected with hepatitis C as children abroad go on to develop potentially fatal liver damage.
Experts had previously thought only one third of people with hepatitis C developed cirrhosis.
The findings from Queen Mary's and the Royal Free hospitals, London, appear in Clinical Gastroenterology & Hepatology.
Hepatitis C is caused by a virus carried and passed on through blood. The infection can go unnoticed for years, but if untreated it can cause severe liver damage, called cirrhosis.
If caught early it can be treated, but if not, it is deadly.
Past studies have only looked at the risks of cirrhosis among people who infected with the hepatitis C virus (HCV) for about 20-30 years, estimating the cirrhosis risk at this time to be about 20%.
Dr Graham Foster, professor of hepatology at Queen Mary's, and his colleagues wanted to gauge what the longer-term risk might be.
They looked at the health of 143 Asian adults who had been infected with HCV as children some 20-80 years ago in Pakistan or Bangladesh.
They worked out how many of these adults developed cirrhosis and compared the rate with that of 239 white patients referred to their hospital.
Infection for more 60 years caused cirrhosis in 71% of HCV Asians, they found.
This prevalence rate was much higher than rate of about 25% found in the study of white patients.
Dr Foster said the difference was mostly likely caused by the duration that a person had HCV.
He said it was likely that the risk of cirrhosis, and therefore death, would also be as high in white people who had been HCV positive for a similar number of years.
"It may be that in Caucasians it will be even worse because they do drink alcohol, whereas the Asian population we were studying were predominantly teetotal," he said.
"This is the first time that we have got a very long-term cohort of a large number of people. We need to look very carefully at this and to change prevention strategies very quickly."
He said that awareness campaigns to prevent, spot and treat HCV infections had been set up, but that it might be important to focus efforts also on ethnic minorities.
"Ethnic minorities may be the group at most risk of dying from hepatitis C," he said.
Dr Foster added an estimated tens of thousands of first generation Asians living in the UK could be at risk, based on infection rates in Pakistan and Bangladesh. Other people known to be at risk include intravenous drug users sharing syringes.
"In East London we are starting a programme in a local mosque where we are going to be screening people for hepatitis C," he said.
Charles Gore, chief executive of the Hepatitis C Trust and president of the European Liver Patients' Association, said the study showed how serious the virus was.
He said it was extremely important to have a "sustained awareness campaign that will get people tested before they develop cirrhosis".
"We have to make very concerted efforts to raise awareness among the Asian community. They are immediately at risk."
A spokeswoman from the Department of Health said its public awareness campaign for hepatitis C, aimed at all sections of the community, began last December.
More information can be found at www.hepc.nhs.uk.