Gay men were given a fresh warning today about the dangers of unprotected sex after the Health Protection Agency reported record levels of HIV infection over the last two years.

The agency said 2,830 cases of HIV were diagnosed across Britain in 2008 after sex between men.

This was less than the 3,050 cases in 2007, but still the second highest annual figure on record.

Rates of infection among gay men have doubled over the past decade and the increase may have been linked to a trend towards less safe sexual practice.

Dr Barry Evans, an HIV expert at the agency, said: "Gay men continue to be the group in the UK most at risk of acquiring HIV and latest national guidelines recommend that gay men should test annually.

He added: "We have taken our eye off the ball a little. We have had some national campaigns and that's been good, but there is not enough health promotion on the back of those campaigns, as we used to have ... There's no evidence of a decrease in the number of transmissions amongst gay men and that's the worrying thing."

The agency said its figures were estimates, based on partial returns from clinics.

HIV diagnoses through heterosexual contact continued to decline from a peak of 5,000 in 2004 to 4,200 last year. Fewer heterosexuals were infected abroad, but there was a small increase in the number diagnosed with HIV after sexual contact in Britain.

The agency said one in five gay men are being tested after the point where treatment should have begun,thus increasing their chances of death within the first year of diagnosis.

Evans added: "Sexual behaviour is a powerful driver and people will look to be intimate sexually so they sometimes feel condoms get in the way.

"There is also a new generation of men in their late teens and early twenties who are experimenting with their sexuality so we need to educate them about HIV."

A spokeswoman for the Department of Health said: "Our national HIV prevention programmes continue to prioritise gay men as the group most at risk of HIV in the UK.

"We provide funding for the Terrence Higgins Trust who are leading a partnership of community-based organisations. They are addressing risk-taking behaviours in gay men, earlier HIV testing, work with HIV positive gay men and those gay men at increased risk of HIV."