The government's Prevalence of HIV in England and Wales report shows that over two-thirds of HIV positive pregnant women do not know they are putting their babies at risk.

And even among gay and bisexual men - the most HIV-aware section of the community - as many as 40% of people attending sexually transmitted disease (STD) clinics do not know they are carrying the virus.

The anonymous surveys have been carried out in prisons, ante-natal units, STD clinics and hospitals around the country between 1990 and 1997.

Future picture

The aim of the surveys is to give health officials an idea of what the future outlook for HIV is in England and Wales.

Drugs can reduce HIV levels, but there is no cure or vaccine for Aids yet

The surveys used leftover blood samples. Specimens were not taken from patients who objected to being tested.

They showed that people in London continued to be at more risk of HIV infection than the general population.

Gay and bisexual men were most at risk. In 1997, one in 11 men who attended STD clinics in London being HIV positive, compared with one in 26 elsewhere.

One in 16 gay and bisexual men under 25 were infected in London.

Just under 3% had been diagnosed since 1996, showing that Aids prevention programmes are not getting through to many people.

Much of the early prevention work was aimed at gay and bisexual men and brought a reduction in the number of new cases.

But health officials fear this may now be wearing off. The survey calls for campaigns to educate "new generations" at risk.

Drug users

However, the message seems to be getting through to drug users outside London.

One in 268 men and one in 245 women drug users outside London are HIV positive.

In London, the numbers are not falling. One in 25 men and one in 66 women in London carry the virus.

Drug users who share needles are at greater risk of getting HIV

Moreover, the number of people sharing needles and syringes - which helps transmit the disease from an infected person - has remained stable in the last five years.

Younger drug users and women are more likely to share needles than others.

Health officials report a rise in the number of acute hepatitis B infections since 1992, which is transmitted in a similar way to HIV - showing the risk is still "substantial".

Heterosexuals

For heterosexuals, one in 125 men and one in 142 women in London who attended STD clinics in 1997 were HIV positive.

One in 801 men and one in 978 women carry the virus outside London.

And saliva tests on inmates at eight prisons show an HIV rate of 0.3% for men and 1.2% for women, although hepatitis rates are as high as 17% for men and 23% for women.

The UK has a relatively low level of HIV infection, compared with other European countries, says the report.

Some 25,000 UK adults - around one in 1,000 people between the ages of 15 and 49 - are HIV positive.

The UK rates are similar to Ireland and slightly more than Germany, Sweden, Norway and Finland, but way below levels in Portugal, Spain, France, Switzerland and Italy.

Around one in 160 people in Portugal aged 15 to 49 is HIV positive.

The report puts the UK's low level of infection down to targeted interventions at groups indulging in high risk behaviour, education campaigns, needle exchange schemes and free and open access to STD clinics.

It calls for a continuation of targeted campaigns, the provision of routine voluntary confidential HIV testing at STD clinics, special services for African women - the highest risk group among pregnant women - and the maintenance of needle exchange schemes.

Public health minister Tessa Jowell said: "The report gives vital information which cannot be obtained in any other way.