Condoms are key to avoiding HIV transmission
Health protection experts estimate there are now 77,400 people with HIV in the UK.
There were more than 7,000 new diagnoses last year - a rise of 6% on the previous year.
Almost a third of people are diagnosed late - meaning they are missing the benefits of early treatment.
Gay men accounted for 41% of new cases, but the Health Protection Agency said heterosexual transmission is steadily increasing too.
Gay men and African people are most likely to have undiagnosed HIV in the UK so we would urge people in those groups in particular to recognise their level of risk and get tested
Lisa Power, of the HIV charity Terrence Higgins Trust
The estimated number of people infected through heterosexual contact within the UK has nearly doubled from 540 new diagnoses in 2003 to 960 in 2007. The bulk of the 4,260 new heterosexual cases were acquired abroad.
The overall figure included an estimated 20,000-plus people in the UK who have HIV, but do not know it.
The estimated numbers of infections acquired through injecting drug use and mother-to-child transmission remained low - 180 and 110 respectively in 2007.
Experts said access to testing must be made easier.
Dr Valerie Delpech, head of HIV surveillance at the HPA's Centre for Infections, said uptake of testing offered in genitourinary and antenatal clinics was good - over a million were carried out last year.
But she said more needed to be done to pick up cases of HIV in the community.
Dr Delpech said: "We need to improve availability of HIV testing in a number of healthcare settings, including general practice, to improve diagnosis of this infection. Without this we will not see the reduction in transmission that we need to see, or a further fall in serious disease."
New testing guidelines, backed by the Department of Health, recommend all men and women between the ages of 15 and 59 in some "high risk" areas of England should be offered a HIV test by their GP.
These areas include 25 primary care trust areas inside London and parts of the South coast, the Midlands, Manchester and Blackpool.
And men who have sex with men - a particularly high-risk group - should be tested annually.
HIV HOT SPOTS
25 areas of London, including Lambeth, Southwark and Islington
Based on primary care trusts where the prevalence of diagnosed HIV infection exceeded two adults per 1,000 population
Lisa Power, of the HIV charity Terrence Higgins Trust, said the fact that so many people were unaware that they were infected with HIV posed a serious threat to public health.
She said: "Not only is this dangerous to their own health, but they are more likely to pass the virus on than someone who has been diagnosed."
"Gay men and African people are most likely to have undiagnosed HIV in the UK so we would urge people in those groups in particular to recognise their level of risk and get tested for HIV regularly."
Overall, 31% of people diagnosed last year were diagnosed very late - perhaps years after infection.
Deborah Jack, chief executive of the National AIDS Trust, said the number of people who should be on HIV treatment but who in fact are not was deeply worrying.
"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."
Liberal Democrat health spokesperson, Sandra Gidley, said: "These figures are very worrying but not surprising. They are the legacy of a government which has left sexual health services to languish by the wayside.
"It is crucial that we ensure future generations are not now put at risk."
Shadow Health Minister Anne Milton, said: "These figures are of huge concern. They are indicative of a generation that was not exposed to the effective tombstone campaigns of the late 1980s.
"These figures are another example of why we in the Conservative Party think it is so important to commit to prioritise public health spending, so that short-term pressures do not compromise the health of the nation in the long-term."