The number of people suffering from severe liver diseases caused by hepatitis C will double in the next 10 years, putting serious strain on transplant services, health watchdogs warned yesterday.

The Health Protection Agency predicted that 10,090 people will be living with cirrhosis or serious liver failure by 2015, of whom 2,540 will die if they do not receive a transplant. The figures compare with 4,855 with serious liver damage last year as a result of hepatitis C infection.

Pete Borriello, director of the HPA's centre for infections, warned that the virus is a "silent killer which kills by stealth" as many people are infected for years without knowing it.

Some 231,000 people are thought to be infected with the virus but only 60,000 cases are centrally recorded. Officials estimate that more have been diagnosed but not reported to the HPA, leaving around 100,000 people unaware that they are carrying the virus.

Hepatitis C is passed on through blood to blood contact. The majority - 92% - of people who have it contracted the disease through sharing needles to inject drugs, and there is a group of people who were infected by blood transfusions before 1991, when screening of donated blood began. Health officials are also recording an increasing number of infections among gay men who are HIV-positive and practise unsafe sex. People who have both viruses have higher concentrations of hepatitis C in their blood and therefore pass it on more readily.

Helen Harris, a hepatitis C expert with the HPA, said: "Hepatitis C is very under-diagnosed simply because people are unaware that they are carrying it. By increasing awareness of the infection, more people will be tested, will receive earlier and more effective treatment, and they can avoid passing it on to others."

The government has launched several information campaigns and backed needle exchange services, but provision is patchy around the country, according to the HPA's annual report on hepatitis, published yesterday. The official number of diagnoses was slightly lower in 2005 compared with the previous year, but officials said that was not significant as there were still thousands of undiagnosed people infected since the 1970s, when injecting drugs became more prevalent. Some 7,580 people were diagnosed last year alone, more than triple the 2,116 diagnosed in 1996.

The virus causes damage to the liver and ultimately liver failure, but progression varies from person to person and can be exacerbated by alcohol consumption. Deaths from all liver disease are rising in the UK - the only country in the world with an upward trend.

It is easily treated in its early stages but rarely diagnosed as it has few symptoms. It is the second biggest reasons for liver transplants after alcohol damage and costs the NHS millions of pounds a year.

Alison Rogers, chief executive of the British Liver Trust, said: "Both hepatitis B and C still remain a low priority for the government. Improving the awareness of hepatitis C is a positive step; however, there is a greater need for the delivery of treatment services. There needs to be better funding for treatment."