The number of people with HIV in the UK is still rising with 53,000 adults now living with the virus, figures show.
There were 6,606 new HIV infections diagnosed in the UK in 2003
More than a quarter - 27% - do not know they have the infection, which could be as many as 14,300 people.
There were 6,606 new HIV infections diagnosed, the Health Protection Agency said, but that could rise to 7,000 once all the data are collected.
The new figures come as the government announced a £300m investment to tackle the UK's sexual health crisis.
The increase in HIV diagnoses compares with 6,017 cases in 2002 and 2,835 in 1998.
The increase means the UK faces rising costs as well as ill health with each HIV infection estimated to cost between £500,000 and £1m in treatment and lost productivity.
Dr Kevin Fenton, author of the HPA's Focus on Prevention report, said the increase in the number of infections diagnosed was "the result of a combination of factors".
"But it is largely contributed to by the migration of people from areas of the world where there is a high prevalence of HIV, such as sub-Saharan Africa," he said.
Of the 6,606 new cases diagnosed in 2003, 58% - 3,801 - were among
heterosexuals while gay and bisexual men accounted for 1,735, or 26%.
The number of new infections diagnosed in gay and bisexual men was the highest for 10 years, Dr Fenton said.
The distribution of HIV cases has changed in the past decade, moving away from gay men and into heterosexual adults.
In 1994, only 31% of new cases were in heterosexuals.
This percentage had almost doubled to 58% by last year.
The number of heterosexual diagnoses likely to have been acquired in this country had increased from 139 in 1998 to 341 in 2003, he added.
Two thirds of HIV infections acquired through heterosexual sex diagnosed in 2003 were in women - 2,465.
Dr Ewen Stewart, HIV spokesman at the Royal College of GPs, said: "It is
worrying that there are now a large number of people in the UK with undiagnosed
"If people are concerned that they may have been exposed to HIV they should
contact their GP or sexual health clinic for a test."
Professor Pat Troop, chief executive of the HPA, said sexual health services were coming under increasing pressure to cope with the number of people diagnosed with HIV and other STIs.
She said: "We carried out a survey looking at waiting times in GUM clinics
which showed that fewer than a third of patients are seen at clinics within the
first 48 hours of seeking an appointment.
"If people are to receive early diagnosis and treatment, these waiting times
need to be reduced."
On Wednesday, the government announced a £300m investment over the next three years in an attempt to deal with the UK's sexual health crisis.