The number of men and women becoming HIV positive as a result of having "straight" sex in the UK has nearly doubled in four years, health officials said today.

The total number of people living with HIV in Britain reached an estimated 77,400 in 2007, yet about 20,000 of those could be unaware that they have the condition, according to the Health Protection Agency (HPA).

Men who have sex with men are still the largest group living with HIV, but unprotected sex between heterosexual men and women accounted for more than half (55 per cent) of the total of 7,734 newly-identified cases last year. These cases are also likely to be diagnosed later, reducing the effectiveness of treatment.

Three quarters of the 4,260 heterosexual cases diagnosed last year were probably infected abroad, but the HPA said that 960 cases originated in Britain in 2007, compared to 540 in 2003.

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The agency said that heterosexual infections within the British population were largely confined to people of black-African ethnicity, of whom two thirds acquired their infection abroad, mainly in sub-Saharan Africa. By contrast, among HIV-infected gay men diagnosed in 2007, 82 per cent probably acquired their infection in the UK.

Experts cautioned that all adults should use a condom with a new partner until both had been tested to prevent the spread of all sexually transmitted infections (STIs).

Valerie Delpech, the HPA’s head of HIV surveillance, said that the patterns of heterosexual transmission of HIV were “mixed”, but black Africans living within the UK are one of the main groups targeted by prevention strategies.

“Large numbers of young people in Britain are increasingly being diagnosed with STIs such as chlamydia, due to unsafe heterosexual sex in the community but HIV tends to affect specific groups,” she said. Newly diagnosed cases “may have been infected outside the UK after travelling to a high-prevalence area, such as sub-Saharan Africa, or had sex with someone from such an area".

She added: “The message is to use a condom with a new sexual partner until both of you can be sure that you are not at risk.”

The number of new diagnoses of HIV in 2007, increased slightly on the 6,840 in 2006, but the HPA said that the rate of cases seemed to have reached a "plateau" in recent years.

On average, there are two HIV positive adults for every 1,000 people in the UK, but this figure is particularly high in London, some parts of the south coast and other inner city areas. The area of highest prevalence is the London borough of Lambeth, where nearly 12 per 1,000 people are affected.

With modern anti-retroviral treatments, most people who become infected with HIV (human immunodeficiency virus) can expect to live for decades after diagnosis, and many never see their infection culminate in Aids (acquired immunodeficiency syndrome). But heterosexuals are more likely to be diagnosed later, or after the point at which they should have started treatment, Dr Delpech said.

A total of 42 per cent of heterosexual men were diagnosed late, and 36 per cent of heterosexual women compared to 19 per cent of gay or bisexual men, making them up to 10 times more likely to die within a year of diagnosis.

Dr Delpech added: “Diagnosing HIV infections earlier will reduce transmission of this infection as those unaware of their positive status pose a greater risk to future sexual partners.

“Late diagnosis also has a major impact on disease and life expectancy and it is vital that people are diagnosed early.”

The National Aids Trust (NAT) also highlighted with "growing concern" that almost one in five people with diagnosed HIV were choosing not to start - or to delay starting - treatment, even when it was recommended that they do so. Dr Delpech said that this could be because some patients could go into denial when diagnosed with HIV.

Deborah Jack, Chief Executive of NAT, said: “Each year a significant number of people are diagnosed with HIV, showing we still have much more to do to reduce ongoing HIV infection in the UK. Funding for prevention and testing must be increased and the Government must being to take seriously the public health challenge of HIV in the UK, as it is growing each year.

"Most worrying is the number of people who should be on HIV treatment but who in fact are not - whether because they are unaware of their infection or because they are opting not to start treatment when recommended.

"Treatment for HIV has revolutionised the condition and people with HIV can now expect a good life expectancy if they are diagnosed early and take their medication as advised. By not getting treatment people are risking their health.”

Lisa Power, of the Terrance Higgins Trust, added: “Gay men and African people are most likely to have undiagnosed HIV in the UK, so we’d urge people in those groups in particular to recognise their level of risk and get tested for HIV regularly.

“If you know your HIV status you can take steps to safeguard your health and that of others.”

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