A quarter of UK sex health clinics cannot treat patients needing urgent help within the recommended 48 hours, the BBC's Panorama programme has found.
One in four clinics could not see patients in the recommended time
The programme contacted 269 clinics and found that, in some cases, patients could wait weeks to be seen.
Waiting times have grown so much that one clinic was taking bookings nine weeks in advance.
The Department of Health said more needed to be done, and it was investing £300m over the next three years.
One of the most senior sexual health clinicians in the country, Professor George Kinghorn, told Panorama that the situation amounted to a "public health crisis".
Gill Bell, Britain's only nurse consultant in sexual health, described how trying to get seen has become "the survival of the fittest."
Panorama said its investigation revealed the scale of the sexually-transmitted infection (STI) epidemic in the UK, and showed a nation where sexual health is in rapid decline, due to big increases in the infection rates for some of the most common STIs.
It also said it shows how the rate of infection is overwhelming our sexual health service.
In the last decade, recorded cases of gonorrhoea and HIV have more than doubled and syphilis is up 1500%.
And the number of sexually active people under 25 infected with chlamydia - the most common STI - is now thought to number just under 500,000.
The programme found 27% of clinics could not treat a patient reporting serious symptoms of an STI within 48 hours.
And only a quarter of clinics offered some form of walk-in service for all patients.
One in five also operated a restrictive system where they had either stopped taking new patients, or only offered a limited number of appointments available for booking at specific times of the week.
In one case the Panorama team were told, by a member of staff at one clinic, to complain to their MP about the situation:
Clinic: "There's no provision for sexual health in London from Friday afternoon to Monday morning."
Caller: "Nowhere at all?"
Clinic: "Nowhere. Nowhere. And if you think it's scandalous as I do, write to your MP."
The programme follows the story of several patients seeking treatment for STIs, including 24-year-old Sian, who developed chlamydia over two years ago and has been struggling to get pregnant ever since.
The Panorama team followed her in to the operating theatre where her surgeon, Professor Bill Ledger, discovers that her fallopian tubes were blocked.
The report comes as the government is due to unveil a £50m advertising campaign - the biggest sexual health campaign since the 1980s - in the next few months warning of the dangers of unsafe sex.
A Department of Health spokeswoman said the government was unsure how the survey had been conducted and whether it had been independently verified.
But she added: "We accept that a lot more needs to be done to reach our goal of offering everyone who needs one an appointment within 48 hours.
"But that is why we've committed over £300m over the next three years for sexual health."
Panorama's "Love Hurts" is broadcast on Sunday 16 October 2005 at 22:15BST on BBC One.
It is also available online at bbc.co.uk/panorama in broadband, live and on demand.