Tucked down a backstreet, Patrick French’s workplace is identified only by a generic blue NHS sign. Nobody would know why the men and women entering the building off Tottenham Court Road in central London were here. Even inside, it’s not obvious. With its blond wood floors and potted plants, all that distinguishes the Mortimer Market sexual health clinic from a dentist’s waiting room is the presence of enormous posters advertising Liquid Silk lubricant.

Such invisibility helps protect patients’ privacy in this most intimate field of medicine. But it also means a crisis can fly beneath the radar. When patients are waiting on A&E trolleys, we all hear about it. When they’re queueing out of the door to be tested for sexually transmitted infections (STIs), we don’t. “People say the NHS has been protected against cuts – well there’s one bit that hasn’t,” says French, a genitourinary medicine (GUM) consultant at central and north west London trust.

One colleague saw a woman with herpes; she only got to see someone at the sixth attempt to get into a clinic

If you think you don’t know anyone who uses a service like this, you’re probably wrong. Last year, almost half a million cases of STIs were recorded in England and Wales, while clinic attendances rose by 13%. The most common diagnosis was chlamydia – easily treated with antibiotics, although it can cause pelvic pain and infertility if left. But what is ringing alarm bells is a rise in cases of gonorrhoea, up tenfold since 2008, and syphilis, an infection that had virtually been wiped out in Britain but is now running at levels not seen since the second world war. The rise is mainly among men who have sex with men, but not entirely. The Victorian spectre of babies born with syphilis is back, with three newborns infected by their pregnant mothers last year.

“When I started working in an STD clinic in 1988, syphilis had been eradicated in Britain. It took 18 months before I saw a single person with syphilis for the first time. Last week, we saw five or six in a day,” says French, who also works with the British Association of Sexual Health and HIV. “It’s the same with gonorrhoea; it became rather uncommon with the advent of HIV. And now it has become really common. Something really dramatic has happened.”

A poster warning about syphilis in 1900. Photograph: Alamy

Crucially, it’s happening inside a service already creaking at the seams. French estimates the sexual health budget across the four London boroughs his trust serves has been cut by 25% in 18 months, with clinics closing in Haringey and Barnet. Patients are travelling further and waiting longer. At the nearby Dean Street clinic, in Soho’s red light district, the 400 appointment slots released daily at 8am are filled within half an hour, leaving 1,000 people still trying to be seen. “One colleague saw a woman with a horrible first episode of herpes; she only got to see someone at the sixth attempt to get into a clinic and by then she had real problems,” he recalls. “From a service where people were guaranteed to be seen in 48 hours to one where people aren’t getting seen – that’s happened in three or four years. I’m concerned that some people aren’t getting care at all.”

Clinics have closed across the country, including in Suffolk, East Sussex, Cambridgeshire and Nottinghamshire. Others are cutting hours or restricting services to younger women only, which saves money. In Liverpool, a clinic operated by the Brook Advisory Centre, a charity offering contraceptive advice and STI screening for young people, recently saw a sudden influx of students; it turned out the university’s sexual health service was so swamped it had temporarily shut its doors.

“In the past, doctors have worked more than their [contracted] hours to see everyone they need to see,” says Lisa Hallgarten, head of policy and public affairs at Brook. “But now there’s no possibility that they are going to see everyone. For doctors, that is very, very uncomfortable.” The fear is that people without obvious symptoms won’t persevere and will unwittingly infect others. French adds: “It’s not the people we see. It’s the people we don’t.”

What makes STI clinics almost uniquely vulnerable compared with other healthcare is that six years ago, the then health secretary Andrew Lansley’s Health and Social Care Act moved the public health budgets that pay for them from the NHS (shielded from austerity) to local councils (which weren’t). Then, to cap it all, George Osborne cut the public health grant to councils by 7.4% in 2015. Almost half of councils in England and Wales now plan to cut sexual health funding, according to a recent BBC survey. In May, an open letter to the then health secretary, Jeremy Hunt, from patients’ and doctors’ groups warned cuts were “undermining the delivery of effective STI prevention, testing and treatment services” and limiting patient access.

Yusef Azad, the director of strategy at the National Aids Trust says local authorities are in a difficult situation. But he suspects it is no accident they’re cutting services for users uniquely reluctant to go public in protest. “You don’t see people outside a council with a placard saying: ‘I regularly get gonorrhoea,’ do you?”

Meanwhile, the requirement for councils to put services out to regular competitive tender means that NHS trusts, charities such Brook and private companies such as Virgin Healthcare are scrapping over contracts; services once provided in a single location are fragmenting.

“It’s quite possible now that a woman will walk into a surgery wanting to discuss her contraception, have her smear test and get an STI screen and be told: ‘We can do your smear test, but we can’t fit your coil because the funding for that has gone to a clinic down the road and we can’t do your screening because the money’s in a separate clinic,’” says Helen Stokes-Lampard, the chair of the Royal College of GPs.

‘The greatest challenge remains the behaviour of people, not bugs.’ Photograph: Stefano Oppo/Getty Images

“And that’s where STI screening misses out because if you’re just worried and want a checkup, you’re likely not to bother if they make it too difficult.” Stokes-Lampard, a practising GP from Lichfield, Staffordshire, says her chief concern is for people in rural areas who can’t easily travel when services move.

Steven Fryett knows how that feels. Eighteen years ago, he was diagnosed with HIV at a time when, as he says, it was “a taboo, really. A lot of things were kept close to chests.”

It was a comfort to discover his local sexual health clinic at St Peter’s hospital in Chertsey, Surrey. The 59-year-old describes the Blanche Heriot unit as a “safety net” where his life was saved at least once.

But last year, it shut after central and north west London NHS trust won the sexual health services contract tendered out by Surrey county council on a tight budget. His nearest clinic is in Guildford, which for Fryett – who was also subsequently diagnosed with cancer – would mean taking three buses just to have his viral loads checked. He worries about the implications for young people who can no longer slip off for an STI test in their home town. “I used to go up to St Peter’s and the GUM clinic would be full of 20-year-olds, just back from their holidays. Saying to them: ‘Hop on a bus and go to Guildford’ – they’re not going to do it.”

Yet this isn’t a simple story of cuts causing a crisis – or, at least, not yet. Although one in three English councils have cut contraceptive services, there is no obvious surge in unwanted pregnancies: conception rates generally and teenage pregnancies are at a record low.

New cases of HIV are falling, thanks to drug regimes reducing viral load so drastically that patients don’t pass it on, even during unprotected sex. The vaccination of teenage girls against HPV, the virus behind cervical cancer and genital warts, means diagnoses of the latter are tumbling. Even the rise of gonorrhoea and syphilis predates Lansley’s reforms, since they started creeping back in the early 00s. Yet experts are concerned about the combination of a decades-long shift in sexual behaviour, emerging risks and funding cuts.

In March came the news nobody wanted to hear: the first case of super-resistant gonorrhoea, in a British man who caught it from a woman in south-east Asia, that was impervious to both the drugs normally used against it. For years, doctors have been seeing cases resistant to at least one antibiotic, but this was the first in which neither frontline treatment worked. “They had to go for a very weird antibiotic I’d never even heard of,” says French. “Fortunately, it seems this person hadn’t transmitted it on, but gonorrhoea is a very clever bacteria.” It can, he says, acquire resistance from other surrounding bacteria if the host has previously taken antibiotics for other conditions. The British Association of Sexual Health and HIV recently warned that another little-known bug that causes genital pain and bleeding, mycoplasma genitalium, is also developing antibiotic resistance.

But the greatest challenge remains the behaviour of people, not bugs. You need only to be unlucky once to catch an STI. Bacteria don’t discriminate by class or age – which is why later-life divorce has brought nasty surprises for some middle-aged people unleashed on to a dating scene.

We have been relying on the fear of HIV to control sexually transmitted infections for the last 30 years

That said, the highest-risk groups are straight people under 25, men who have sex with men and black ethnic minority communities. And STIs don’t just increase for no reason. “We have got to look at things like people having more partners, different kinds of mixing of partners, increased risk-taking and overlapping relationships,” says French. “That’s particularly an issue with men who have sex with men – and that’s where the biggest increase is.”

Men attending his clinic are now routinely asked about “chemsex” drugs such as GHB, mephedrone and crystal meth – taken to reduce inhibitions – which are linked to riskier behaviour including partner-swapping, group sex and ditching condoms.

The awkward question, however, is whether recent breakthroughs in HIV have had unexpected consequences. The NHS recently began a trial of “pre-exposure prophylaxis”, or Prep, a drug which, when taken daily, protects against HIV infection. Some gay men had been buying it privately online, but now 10,000 people are getting it free. While it is too early for the full impact to be showing in official statistics, new HIV cases in French’s clinic have almost halved in little over a year.

It may be a life-changing discovery. But, just as women whose main fear is pregnancy rather than infection may stop bothering with condoms when they go on the pill, could the peace of mind Prep offers breed complacency about infections other than HIV?

Studies in Canada and Los Angeles have shown a slight increase in STIs among men taking the drug, although early studies in London didn’t. French’s hunch is that “a lot of the concern around HIV transmission has been reduced and I’m sure that’s going to play into the increase in STIs”.

The 1986 Aids awareness campaign. Photograph: News (UK) Ltd/Rex

But that’s not, he stresses, an excuse to sit in judgment on gay men, who are, if anything, more reliable condom users than straight people. “My take is that gay men who are taking Prep are being really responsible, in terms of their sexual health. But, undoubtedly, we are seeing an increased rate of syphilis, particularly among gay men. I think that’s a much more complicated issue than just saying: ‘People are being reckless.’”

Even if Prep is changing behaviour, he points out, that doesn’t explain why STIs were rising before it reached the market. What is more likely is that receding fear of HIV has left a hole in public health strategies for gay and straight couples.

Between 1986 – when the “Don’t die of ignorance” ad campaign was launched – and 1993, the percentage of young British women aged between 20 and 24 who said their partners used a condom doubled. In the early 90s, STI rates dipped accordingly.

But by the 00s they were rising again. By 2008, over half of men and women who weren’t in exclusive long-term relationships said fear of infection wasn’t changing their behaviour. “If we’re honest, we have been relying on the fear of HIV to control sexually transmitted infections for the last 30 years,” says Azad. “We have to persuade people that it is worth taking precautions against STIs, rather than rely on HIV to do all the heavy lifting.” What is needed, he says, is a positive ad campaign acknowledging that people enjoy sex, but making them want to do it responsibly.

In the case of syphilis, which unlike HIV is easily spread by oral sex, that doesn’t just mean condoms. Regular testing prevents infection spreading and the advice is that sexually active gay men should screen annually, people from black ethnic minority backgrounds (at statistically higher risk) should screen regularly if having condom-less sex with new or casual partners, and young heterosexual people need an annual chlamydia check; everyone, ideally, should get checked when changing partners. It’s unclear, however, if the system would cope if everyone took that advice.

In an attempt to relieve pressure on overcrowded clinics, some London boroughs are now rolling out DIY home testing. People seeking screening will be sent a kit to collect their own swabs and blood samples and post them back; only positive testing will lead to an appointment, saving clinic time. It is, says French, a reasonable solution to a difficult situation But what about people who don’t want a test kit through the letterbox?

“You often get them late afternoon on a Friday. If somebody doesn’t want to go home, that’s when you get these conversations,” says Alison Hamnett, director of operations across the north for Brook. They may start with asking for free condoms, but eventually the real story emerges: sexual exploitation, abusive relationships, precarious lives. Girls who don’t even feel entitled to refuse sex, let alone insist on protecting themselves.

Some are guarded. “Particularly if they are being groomed, they will have the answers to the questions down pat,” says Hamnett. “But the receptionist will say she saw a car outside drop them off – and the same car is coming with lots of young girls …” Posters hanging in the waiting room of the Manchester clinic where we meet explain the difference between exploitative and loving relationships: no, it’s not OK if he offers a roof over your head and expects sex in return.

The drive to save NHS money has had a huge impact on waiting times. Photograph: Alamy

The Burnley, Blackburn and Oldham clinics tend to see more grooming-gang victims, says Hamnett. In Liverpool, she found them dealing with a young homeless man, released from prison, who had been having sex in broad daylight in a car park while intoxicated. Manchester saw a young Muslim girl who was being radicalised. The checklist used with clients ranges from female genital mutilation to mental health issues. “We had a young woman of about 17, very intelligent, got all her A-levels and went to university,” says Hamnett. “She was bipolar and, when she was on her meds, she was great. When she wasn’t, she’d sell herself for sex.” The clinic helped her until she was too old to use its service, which is restricted to under-19s. They don’t know where she is now.

Brook’s expertise is in this area – where sexuality, deep-seated social problems and mental health issues collide – and is, says Hallgarten, what makes them “very good value for money”, as identifying the root cause of sexual risk-taking offers more chance of changing it.

But specialist clinics for vulnerable young people such as these are increasingly merging with more general services to save money. There is a push, says Hamnett, towards using GPs instead for contraception. That may work for young people with happy sex lives, but there is a reason appointments here last for up to 40 minutes, not the 10 minutes a busy GP might offer. “I feel as if we’re almost waiting a few years down the line for teenage pregnancies to go up,” she says ruefully. It is this sense of a clock being turned back that worries many.

French started out working from a prefab in a hospital car park as a newly qualified doctor. “It all changed in 2002 when the NHS was funded properly – to be going from five, six weeks waiting to be seen in an STD clinic to being seen in no time at all. That all happened between then and 2008 and to see that deteriorate so quickly is heartbreaking. We didn’t know how good things were until it disappeared.”

Brook offers confidential advice on issues raised in this article. Further information: brook.org.uk. The NHS website has full details on the sexual health services available