The number of Britons expected to be diagnosed with HIV in the next three years is likely to rise sharply, according to figures released yesterday.

Estimates based on existing trends suggest that the 23,000 people living with HIV in 2000 will increase to 34,000 by 2005, an increase of 47 per cent.

In addition, another 10,000 people might not know that they are infected with the virus and so be at risk of transmitting it to their sexual partners.

Scientists from the Public Health Laboratory Service (PHLS) in north London said t many newly diagnosed patients would have been infected for several years, so the increase did not indicate a worsening of the HIV epidemic.

"These figures are not all doom and gloom. To some extent, of course, they are encouraging, because some of the diagnoses are in people who were infected years ago but are now coming forward, getting dignosed and then receiving treatment," said Barry Evans, head of the HIV division at the PHLS's Communicable Disease Surveillance Centre.

"But there is a very worrying trend here. There is no doubt that new infections are occurring for a disease which we know how to prevent but for which we have no cure," Dr Evans said.

The latest figures from 2001 show that there has been a 17 per cent increase in HIV diagnoses compared with the same time last year. There were 3,342 new HIV cases up to the end of December, compared with 2,868 at the same point in 2000.

By the time all the HIV diagnoses for 2001 are reported to the PHLS, Dr Evans believes the final total will reach more than 4,000 – the highest number of new HIV positives since the epidemic began.

The bulk of the new cases are people who have acquired the virus abroad, mainly in Sub-Saharan Africa, but they also include a sizeable number of heterosexuals who have become infected in Britain.

Last year, 1,095 gay or bisexual men and 1,758 heterosexuals were diagnosed with HIV.

"Although gay and bisexual men remain overall the biggest risk group, this is the third year in a row where the number of heterosexual diagnoses exceeded diagnoses in gay men," Dr Evans said.

"In part this reflects initiatives to encourage heterosexuals, who may previously not have thought of themselves as at risk, to consider HIV testing."

Dr Evans said that the latest figures underlined the importance of the safe-sex message to both men and women. "It is vital that within the new strategy we reinvigorate the 'safer sex' message," he said.