Charlie Sheen learned of his HIV diagnosis four years ago

Following weeks of speculation about his condition, actor Charlie Sheen has publicly revealed that he was diagnosed with HIV "roughly four years ago".

The 50-year-old today claimed that he had been blackmailed for millions of dollars over his illness and that he would stop paying the extorters now that his HIV status was public. One prostitute had even taken a photograph of his antiretroviral medication and threatened to sell it, he claimed.

Speaking on NBC's Today show, he said: "I have to put a stop to this onslaught, this barrage of attacks and of sub-truths and very harmful and mercurial stories that are about me, that threaten the health of so many others that couldn't be further from the truth."

Sheen, who has previously boasted about sleeping with thousands of women and has been in and out of rehab for drink and drugs, said it was "impossible" that he could have transmitted HIV to anyone since his diagnosis. He added that they were a "hard three letters to absorb".

The actor said he was "not entirely" aware of how he contacted the virus, but his doctor confirmed that he did not have Aids.

There has been much speculation about Sheen's health in recent weeks since show-business site Radar Online claimed a Hollywood "megastar" was HIV-positive.

Sheen, the son of actor Martin Sheen, and star of Hot Shots and Anger Management, said his infamously strange behaviour in 2011 was not due to the diagnosis. He was dropped from his long-running role in CBS sitcom Two and a Half Men amid a public meltdown, in which he criticised the show's creator and claimed he had "tiger blood" and "Adonis DNA".

Today, Sheen said he wished he could blame the episode on his HIV diagnosis, but said that it was "more roid rage".

According to the US Centers for Disease Control and Prevention, an estimated 1.2 million people in the US have HIV, with 50,000 more people contracting the disease each year.