The National Gay Men's Sex Survey found that 77% of gay and bisexual men have been tested for HIV, which is more than in previous years.
Cary James head of health promotion at the Terrence Higgins Trust - who commissioned the survey - said this is particularly concerning, as knowing whether you have HIV or not is "key to tackling the HIV epidemic".
In addition to understanding how many men were being tested for HIV, the survey found that only 60% of participants were happy with their sex life - with over 65s being most likely to be happy and men in their 40s being least likely to be happy.
James added: "We're concerned that a third of gay men are not definite about their HIV status - particularly as we know that one in seven men who have sex with men are undiagnosed.
"Knowing your HIV status is key to tackling the HIV epidemic, as people who are on treatment are highly unlikely to pass on the virus, so it's really important to get tested. Testing is fast, easy and confidential."
Lead researcher Dr Ford Hickson, of the London School of Hygiene and Tropical Medicine, said: "Our findings show that not all men having gay sex are accessing sexual health services regularly. Half of the men we surveyed didn't know that doctors in the UK recommend they test for HIV at least once a year.
"More positively, the vast majority of men are confident they could access an HIV test if they chose to, and HIV testing services in England are very highly regarded among gay and bisexual men."
The survey flagged that many people are uncertain around HIV and how it is transmitted.
One in five men were not confident that deep kissing cannot pass on HIV, and one in four were not aware that effective HIV treatment reduces the risk of HIV being transmitted.
Cary James said: "Myths about how HIV is transmitted create fear and stigma which can discourage people from finding out their HIV status.
"These findings reaffirm the need to normalise HIV testing and bust the most common misconceptions."
The survey also discovered that one in three sexually active gay men have had unprotected anal sex with at least one non-steady partner in the past year.
James added: "Studies have suggested that condoms have prevented 80,000 infections since the start of the HIV epidemic. But this survey shows that perfect condom use is not a reality for everyone.
"We must continue to champion safe sex messages to gay men, but we also need to tackle prevention on all fronts if we are to beat this epidemic. That means regular testing, successful treatment and, critically, PrEP (where people at high risk for HIV take medication daily to lower their chances of getting infected) - in addition to condoms."