An actor has claimed he is to blame for a syphilis outbreak that has stopped filming in the US porn industry.

Mr Marcus has said he altered a test result so he could still work because a doctor assured him he would be safe after taking penicillin.

Industry body The Free Speech Coalition earlier this week called for a halt on filming after nine cases of the STI came to light.

As a precaution, doctors have advised all US porn actors to get tested.

Mr Marcus, who's been performing in adult films for 18 years, told porn industry news source Xbiz.com that he was "very sorry" for altering the results of his test.

Every doctor I talked to up to that point had said get the shot. That's it Porn actor Mr Marcus

He claimed his doctor had informed him he would not be infectious if he took one shot of penicillin and waited 10 days before having sex.

"Once I did that shot, I felt pretty confident," said Mr Marcus. "Every doctor I talked to up to that point had said get the shot. That's it."

The actor said he waited the recommended time, but that a test on 21 July showed the infection was still "reactive" in his body.

'I felt okay'

He told Xbiz.com that the doctor then reassured him it wasn't dangerous and that his body would always show a base level reading for the infection.

"At that point, I felt okay that I can continue to work because he said it's just always going to read that way," said the porn actor.

Believing he was not infectious and keen to work, he said he photocopied the STI test, leaving out the part that showed the syphilis information.

It's thought he then worked three times using the altered test between 24 July and 8 August.

The female performers in the films have been contacted and have since re-tested negative, said industry body The Free Speech Coalition (FSC).

The FSC said it was now trying to trace the people Mr Marcus had worked with before he first discovered he was infected.

We've talked to the nation's utmost expert on syphilis. So we've been pulling in all the experts The Free Speech Coalition

Infected actors will be able to go back to work 10 days after taking penicillin.

In August 2011, the US porn industry again stopped production after an adult film performer tested positive for HIV.

A new law was passed in January 2012 requiring that Los Angeles porn companies ensure their actors use condoms.

Later this year voters will get the chance to extend the law to the wider LA County area.

Critics say such a law would push production underground and increase health risks.