Health protection experts estimate there are now 77,400 people with HIV in the UK...
Almost one fourth of the people with the virus are probably unaware of it. The overall figure included an estimated 20,000-plus people in the UK who have HIV, but do not know it.
Experts said access to testing must be made easier.
Dr Valerie Delpech, head of HIV surveillance at the HPA's Centre for Infections, 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."
Gay men = high risk
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.
Lisa Power, of the HIV charity Terrence Higgins Trust, said: "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."
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."