A survey has suggested the majority of sexual health clinics have turned patients away in the last year.
There are fears poor access to care could fuel the spread of STIs
The snapshot picture from 69 doctors from across England found two-thirds said their clinics could not cope with demand.
The sexual health charities who carried out the survey said £300m of government investment last year had had an impact.
But they warned the money was not getting through to frontline services in some areas.
The survey is the third yearly analysis of services to be carried out by the Terrence Higgins Trust, the British HIV Association BHIVA) and Providers of Aids Care and Treatment (PACT).
It found that 34% of clinicians reported they had "often" turned people away without being able to offer them any help.
Another 30% said they had "occasionally" had to do so.
The charities warn most of these patients would have been unlikely to have been able to access sexual health services elsewhere.
They said this meant those who did have a sexually transmitted infection would have continued to experience symptoms themselves, and potentially pass their STI on to new partners.
One specialist who responded to the survey said: "We have a single centre covering 500,000 people - currently turning away 600 patients a week."
More than half of the specialists said their ability to provide services has got worse over the past year.
The survey also revealed long waits for tests.
It suggested one in five patients wait a month for an STI test and more than a third wait more than two weeks for an HIV test.
The charities also surveyed 47 primary care trusts.
Almost half said they had not increased their spending on sexual health in the last year.
But they said government investment had improved services - where funding had reached clinics.
Lisa Power, head of policy at Terrence Higgins Trust said: "Despite the government's commitment to improving sexual health, many PCTs and clinicians are still struggling to improve access to diagnostic and treatment services, and sexual ill-health continues to worsen in England.
"Where government money is getting through to sexual health services, matters are improving. But too often, PCT managers are failing to take sexual health seriously."
She said last year's White Paper on Public Health had committed the NHS to cutting waiting times in sexual health clinics and a national campaign to improve public awareness about safer sex.
But she added: "This needs to be matched by commitment at local level if we are to see better sexual health across the country."
A Department of Health spokeswoman said: "This is a small survey that only looked at 15% of PCTs.
"Tackling the rise in STIs is a government priority. And that is why we have just invested an extra £300m for sexual health."
She said the aim was for all patients who needed an appointment would be seen within 48 hours.
She added: "One of the aims of our strategy is to relieve the burden on traditional services by providing screening and testing in a range of different settings such as pharmacies, colleges and GP surgeries."