Sexual health experts are seeing a dramatic increase in the number of cases of syphilis - fuelled by unprotected gay sex and an outbreak among mature women who are suspected of "swinging".

Official figures released yesterday show that the highly infectious sexually transmitted disease is making a comeback 10 years after doctors believed it had been eradicated.

The rise - 23% last year on top of 39% between 2003-04 - is partly attributed to the gay community practising less safe sex than in the 1990s, according to experts at the Health Protection Agency (HPA). But among women the rise has been partly caused "by a couple of outbreaks in particular sexual circles among more mature women", said Professor Peter Boriello, director of the HPA's centre for infections.

He refused to confirm that this meant "swingers" and the HPA later insisted it did not have the evidence to confirm this. But it is not thought to be solely due to prostitutes. "Historically, syphilis often involves sex workers but we don't believe that's the primary driver here," said his deputy, Dr Barry Evans, a consultant epidemiologist and expert on HIV.

The figures for syphilis, which relate only to cases treated at genitourinary medicine (GUM) clinics, show that, while the number of cases remains relatively low - 2,807 last year compared with 2,278 the previous year - they represent a 1,159% increase since 2000, and a 2,048% increase over the past 10 years.

The dramatic rise has been seen most clearly among the gay community, leading to fears that messages about safe sex, so prevalent in the mid-1990s, have been eroded. Sixty per cent of the men infected last year were gay, with 1,445 new cases, compared with 20 in 1996. Oral sex accounted for around 40% of new cases.

Heterosexual men have also seen a steady increase - with 945 cases compared with 74 in 1996 - while cases among women have risen from 283 to 417 in the past year.

Congenital syphilis - syphilis in babies, who are born with severe abnormalities - is also beginning to recur, indicating the return of a disease which if untreated can lead to blindness, brain disorders, heart problems and death. "We're seeing the occasional congenital syphilis cases and that means women of childbearing age are acquiring it. The numbers aren't [high] but we are seeing this," Dr Evans said.

The figures came as the HPA revealed the number of sexually transmitted infections (STIs) had increased by 3% over the last year, to 790,387, with chlamydia, the most commonly diagnosed STI, rising by 5% to 109,832.

New cases of gonorrhoea bucked the trend by falling by 13% - a substantial drop for the second successive year - although they still rose in the gay community.

Sexual infection disorders have been almost continually rising since the 1990s and yesterday's figures show the highest increase for both sexes is in the 16-24 age group.

Prof Boriello suggested young people tended to view STIs flippantly and that the serious consequences needed to be conveyed. "'It's a dose of the clap - you go and get an antibiotic, so what's the big deal?'

"The big deal is HIV severely disrupts the immune system and can kill; human papilloma virus can give you cancer; chlamydia can make you sterile; syphilis can cause brain disease and can cause congenital malformations - and that's just a sample. We need to shatter the complacency that exists among young people."

The figures also indicate that the total workload in GUM clinics has risen by 9%, at a time when almost half (46%) of patients are still unable to gain access within the government's 48-hour target.

The sexual health charity Brook, and the Family Planning Association, said the figures illustrated the desperate need for investment in sexual health services.

Will Nutland, head of health promotion at the Terrence Higgins Trust, said there had been a lack of investment in sexual health education for gay men. "Gay men are leaving school without even the most basic information on sexual health," he added.

The figures also came ahead of a report by the Men's Health Forum, calling for the government to target young men in the chlamydia screening programme. At present only 17% of those screened are men, although the incidence of the disease is as high as among women.