HPV infection is the leading cause of cervical cancer

A study by the Health Protection Agency (HPA) found many teenagers are infected with at least one strain of the human papillomavirus (HPV).

The researchers found the risk of infection is "substantial" by the age of just 14.

A vaccination programme for girls as young as 12 is set to begin next year.

The plans have attracted criticism by those who claim the programme will encourage promiscuity, but advocates have argued that it is necessary to start early to maximise the impact on public health.

HPV, which is transmitted through sexual contact, is responsible for most cases of cervical cancer, which kills more than 1,000 woman a year in the UK.

The HPA researchers tested blood samples from 1,483 females aged 10 to 29 for signs of HPV infection.

The results, published in the British Journal of Cancer, showed that by the age of 18 around one in five females show signs of infection, and by the age of 24 the figure climbs to around 40%.

Not compulsory

Professor Pat Troop, HPA chief executive, said: "This study is a valuable addition to our understanding of HPV infection in women in England and should contribute to effective policies to prevent genital warts and cervical cancer.

"With the government's recent announcement of the possible introduction of HPV vaccination, such research will help us and other public health experts to determine the impact of HPV vaccination."

The Department of Health, acting on a recommendation from a committee of experts, announced plans in June to start a HPV vaccination programme next year.

It is likely that girls aged 12 to 13 will be offered the vaccine in three doses over a six-month period. There will be no compulsion.

At present, there are two vaccines designed to be used in a vaccination programme.

Gardasil, made by Merck and Sanofi Pasteur, has already been approved in many countries, while Cervarix, mae by GlaxoSmithKline, is expected to be launched in Europe later this year.

Latest data on Gardasil shows it protects against a total of 14 different strains of HPV, all of which can cause pre-cancerous lesions.

It provides almost complete protection against HPV types 16 and 18 - which together cause 75% of cervical cancer worldwide.

Sex education

Juliet Hillier, of the sexual health charity Brook, said of the latest research: "Statistics like this demonstrate a real need to improve education and prevention programmes which target young people.

"The government must urgently implement a vaccination programme for girls and boys before they become sexually active and ensure resource is available to do so."

Norman Wells, from the Family Education Trust, said: "These disturbing figures highlight the failure of sex education programmes which tell children that there is nothing wrong with sex at any age so long as they use a condom."

The HPA also released a "modelling" study showing that a vaccination programme would be a cost-effective use of NHS resources if the jab protected girls against cervical cancer for at least 20 years.

Its research showed that up to 70% of cases of cervical cancer and 95% of cases of genital warts in men and women could be prevented by the jab.