The global epidemic of HIV, the virus that causes Aids, is tightening its grip on Britain with a record number of new cases diagnosed last year. Soaring rates of all sexually transmitted infections are fuelling the rise, with an estimated 49,500 people living with HIV in 2002, an increase of 20 per cent on the previous year.

Over 15,000 of these - one third - do not know they are infected, increasing the potential for further spread, according to the Health Protection Agency (HPA) which released the figures ahead of World Aids Day on 1 December. The epidemic in the UK has also speeded up because of complacency among young people about the risks of unprotected sex and the mistaken belief that the disease has been beaten by modern treatments.

There were 5,711 new diagnoses of HIV to the end of September last year, the highest since records began in 1987. The rest of the increase, from a total of 41,700 diagnosed cases in 2001, was due to sufferers' blood being tested without their knowledge.

The number of new cases was 15 per cent up on the 4,982 diagnosed in 2001 and is expected to rise to 6,400 when all the reports are received. The rate of infection has more than doubled since 1997.

Two-thirds of the cases were acquired outside the UK, triggering renewed calls yesterday for immigrants to be screened. The cases' overseas origin - most from sub-Saharan Africa - has changed the nature of the epidemic, with heterosexual cases now outnumbering homosexual/bisexual infections two to one.

Of the 1,850 cases of HIV acquired in the UK during 2002, about 1,500 - 80 per cent - were among gay and bisexual men and 275 among heterosexuals.

Tory health spokesman Tim Yeo said: "We cannot justify ignoring the health of those who apply to live permanently in the UK. Not conducting basic health checks for serious diseases such as TB or HIV both disadvantages the applicant and overloads the NHS."

Sir Andrew Green of Migration Watch, the right-wing pressure group, said immigrants wishing to stay in Britain should be screened before they arrived. "There is a substantial problem in the UK and we should not add to it," he said.

The World Health Organisation is opposed to HIV testing at borders on the grounds that it is discriminatory, but the practice is widespread. At least 70 countries, including the US, Australia, Russia, China and most Arab states insist on HIV testing for anyone intending to settle in the country for more than a few months.

Treatment with antiretroviral drugs costs around £15,000 per year for each patient and there is concern that immigrants with HIV/Aids put an unsustainable burden on the host country's health system, as well as threatening the health of the population. But British experts say testing is useless, citing the failure of aggressive testing regimes in Russia and China to curb epidemics in those countries.

A report from the Institute for Public Policy Research, the Labour think tank, said screening immigrants for infectious diseases would drive the problem underground and increase risks by stigmatising victims. It called on the Government to introduce "welcome health checks" for immigrants, in place of compulsory screening.

Sexual health specialists say warnings about the dangers of unsafe sex have been ignored, causing a huge increase in sexually transmitted diseases since the mid-1990s. Cases of chlamydia were up 14 per cent last year, gonorrhoea up 9 per cent and syphilis up 68 per cent. Women are more vulnerable to being infected and two thirds of the cases were among young females aged 16-24.

Professor Michael Adler, an international authority on Aids at University College Hospital, said: "The original messages are not being remembered and that reinforces the case for universal, mandatory sex and relationship education in schools. We are sending young people out into the world without the right preparation in how to negotiate relationships. Sex education is not just about the mechanics."

Dr Kevin Fenton, a public health consultant at the HPA, said the highest increases in STDs were among gay men, with a doubling of cases of gonorrhoea since 1999 and a rise in syphilis from 52 cases to 607 in the same period.

"The group most at risk of contracting HIV in the UK remains gay and bisexual men, accounting for about 80 per cent of the new diagnoses in 2002 that were likely to have been acquired in the UK," he said. "World Aids Day reminds us that the problems we face with HIV are not going away, despite it being a disease that is largely preventable."

An HPA report, Renewing the Focus, said the rising trend was putting the sexual health services under increasing pressure.

The health department responded by announcing £15 million for STD clinics in England. A spokesman said the Cabinet Office was examining proposals for dealing with the threat of infectious disease linked with immigration and its impact on the NHS.