Huge improvements in treating and testing for HIV have failed to curb infections in gay and bisexual men in England and Wales, a study suggests.
New infections were static at about 2,300 a year between 2001 and 2010, despite rises in early diagnosis and far more people taking medication.
Scientists, writing in the Lancet Infectious Diseases, said there had been a return of risky sexual practice.
Campaigners said there were "real challenges" in HIV prevention.
There is concern about the spread of HIV in men who have sex with other men. The Health Protection Agency's latest report said HIV had reached an "all-time high" in this group, with more and more new cases reported each year.
Working out the number of new infections each year is complicated, as many people are undiagnosed and there can be a long delay between someone being infected and finding out.
Researchers at the Medical Research Council Biostatistics Unit, in Cambridge, estimated when people were infected based on the state of their immune system.
Their study showed little difference between the spread of HIV in 2001 and 2010.
"We found the number of new infections remained quite stable over the decade, there was no evidence of a decline," researcher Dr Daniela De Angelis told the BBC.
In that time the number of gay or bisexual men being tested in clinics went from 16,000 to 59,000 a year, the time from infection to diagnosis fell from four years to just over three and the numbers taking antiretroviral therapy went from 69% to 80%.
Dr De Angelis said: "Perhaps a resurgence of unsafe sexual practice might be fuelling the spread of HIV or perhaps testing is not very targeted."
The Health Protection Agency said it was "seriously concerned" about the level of transmission and that unsafe sex was "the most plausible explanation" for the findings.
The organisation's head of HIV surveillance, Dr Valerie Delpech, said: "Men who have sex with men should get an HIV and STI [sexually transmitted infections] screen at least annually, and every three months if having unprotected sex with new or casual partners - and we urge clinicians to take every opportunity to offer the test to this group."
Sir Nick Partridge, the chief executive at Terrence Higgins Trust, which campaigns on sexual health, said: "These findings highlight the real challenges faced by HIV prevention work, which need much greater attention."
He said that spending on safer sex campaigns had "fallen dramatically" over the past 10 years and "we must challenge the assumptions some gay men make about HIV and re-energise gay communities to tackle a real and growing threat to their sexual health".
Yusef Azad, the director of policy at the National Aids Trust, said: "Around seven gay or bisexual men a day in the UK are getting HIV.
"Prevention services so far have been under-resourced, without a clear focus on outcomes or effectiveness.
"They often do not address the cultural and structural drivers of HIV transmission amongst gay and bisexual men - including drug use, mental health issues and the gay scene."