The findings appear to bolster the case for giving the vaccine to schoolgirls before they become sexually active.

The government has agreed to introduce the vaccine into the immunisation schedule in September 2008 for all girls aged 12 and 13.

For the study, the researchers randomly assigned 2,189 women, aged 18 to 25, who had tested positive for HPV into two groups.

One group received three doses of the HPV vaccine, while the other group received a control hepatitis A vaccine, over a period of six months.

After six months, the average rate of HPV viral clearance was only 33 per cent for the participants who had been given the HPV vaccine, compared with 31 per cent in the control group.

After 12 months, the HPV vaccine was found to be less effective at reducing the infection than the control vaccine.

Those who had received the HPV vaccine had an average viral clearance rate of 48 per cent, compared with 49 per cent for the control group.

Measurements of HPV-16/18 antibodies, cytologic results and HPV viral load were taken from a selection of participants who had been given the HPV vaccine, but no evidence was found to show the vaccine had reduced the virus.

Meanwhile, a UK survey has revealed that only 2.5 per cent of respondents knew that the most common cause of cervical cancer was an STI.