London (CNN) Former Welsh rugby star Gareth Thomas has revealed he is HIV positive and has vowed to help "break the stigma" surrounding the illness.

Thomas is believed to be the first British sportsman to announce that he is living with the virus. He was also the first professional rugby union player to announce that he was gay , when he came out in 2009.

The former British Lions captain, 45, said he was forced to announce his condition after being threatened with blackmail.

"Hello I'm Gareth Thomas, and I want to share my secret with you," he said in a video posted on his Twitter page . "Why? because it's mine to tell you, not the evils who have made my life hell, threatening to tell you before I do, and because I believe in you and I trust you.

"I am living with HIV; now you have that information that makes me extremely vulnerable, but it does not make me weak."

He added that while he is being "forced" to announce his condition, he pledged to "fight to educate and break the stigma" surrounding the illness.

"I am asking you to help me to show that everyone lives in fear of people's reactions and opinions... But that doesn't mean we should have to hide, but to do this, I really, really need your support," he said.

Thomas said in an interview with the Sunday Mirror that he had been living with "this secret for years," and that the "shame" he felt from his diagnosis left him feeling suicidal.

Thomas won a total of 103 caps during his career. He is pictured here during the opening ceremony ahead of a Rugby League World Cup match between Australia and England in 2013.

"I was in a dark place, feeling suicidal. I thought about driving off a cliff," he told the newspaper. "To me, wanting to die was just a natural thought and felt like the easier way out, but you have to confront things."

Thomas said that he will "never forget" the moment that he received his diagnosis when going for a routine sexual health check-up in the Welsh capital, Cardiff.

"I'd had tests every now and again and they'd always come back okay. I didn't feel ill and I thought everything was going to be fine," he said. "When (the doctor) said those words, I broke down. I was in such a state. I immediately thought I was going to die. I felt like an express train was hitting me at 300 mph."

The sporting legend is now set to discuss his diagnosis in an upcoming documentary for BBC Wales. In a trailer for the documentary, Thomas says that he is going public with his condition because he wants to "remember what it is like to live again; I want to remember what it's like to feel free."

Gareth Thomas was the first professional rugby union player to announce that he was gay in 2009. He is pictured here in 2011 alongside former British rugby player, Lawrence Dallaglio.

He said he hoped to empower many people who are in the same situation as him. "I think what I want to learn more than anything, is that I got HIV and it's okay," he says.

Thomas' announcement has drawn widespread praise from beyond the sports world. Prince William wrote on his official Twitter account : "Courageous as ever - legend on the pitch and off it. You have our support Gareth."

Jeremy Corbyn, the leader of the opposition Labour Party, tweeted: "Gareth Thomas has again shown enormous strength in declaring himself HIV positive. A role model challenging stigma and prejudice. His example offers hope and resolve to others."

The Terrence Higgins Trust, a leading British HIV charity, added: "He wants everyone to know that HIV shouldn't stop you doing anything you want to. We're proud to call him a friend."

Thomas told the Sunday Mirror that he now takes one tablet containing four medications each day to treat his illness, and that the virus is now "undetectable" and cannot be passed on.

The former rugby star won a total of 103 caps during his career and scored 41 tries for Wales between 1995 and 2007. He is also ranked 13th on the list of the leading test try scorers in international rugby union matches.

Thomas married his wife, Jemma, in 2002, but they later divorced before he announced that he was gay in 2009. He married his current partner, Stephen, three years ago, who he says does not have HIV.