Champion swimmer Ian Thorpe has revealed he is gay in an interview with Sir Michael Parkinson, after years of scrutiny from the Australian public.

In the tell-all interview that aired on Channel Ten on Sunday night, Thorpe also discussed his battle with depression, which he kept secret from his loved ones.

Thorpe had previously insisted he was not gay, and in his 2012 autobiography wrote: "For the record, I am not gay and all my sexual experiences have been straight."

Speaking with Sir Michael he said he was scared of how people would react if he came out as gay.

He said he had lied about his sexuality in the past, adding: "I'm not straight. And this is only something that very recently, we're talking in the past two weeks, I've been comfortable telling the closest people around me exactly that."

Sorry, this video has expired 'I was already living somewhat of a lie': Thorpe ( Footage courtesy Channel Ten )

The five-time Olympic champion said he had wanted to come out for a long time, but "didn't feel as though I could".

"What happened was I felt the lie had become so big that I didn't want people to question my integrity," he said.

"And, you know, a little bit of ego comes into this. I didn't want people to question that... have I lied about everything?"

"I'm comfortable saying I'm a gay man. And I don't want young people to feel the same way that I did. You can grow up, you can be comfortable and you can be gay.

"I was concerned about the reaction from my family, my friends."

Thorpe said his parents were shocked but supportive.

"I'm pleased to say that in telling them, and especially my parents, they told me that they love me and they support me," he said.

"And for young people out there, know that that's usually what the answer is."

Thorpe said although he does not feel like people had a right to ask questions about his sexuality, his life would have been much easier if he had been honest from the start.

"People will criticise me, some people won't like the idea, other people may applaud me for it. But it's... it's me," he said.

"I'm a little bit ashamed that I didn't come out earlier, that I didn't have the strength to do it, I didn't have the courage to do it, to break that lie.

"But everyone goes on their own path to do this."

Thorpe flagged a potential career in politics - something he has "always" been interested in - and said he would like to have a family with a partner in the future.

"I love kids and I'd love to have a family," he said.

Thorpe kept depression from friends and family

Thorpe has struggled with depression for much of his life, and was admitted to a rehabilitation facility earlier this year after using antidepressants and medication for a shoulder injury.

Thorpe told Sir Michael he kept his battle private for many years, and at times turned to alcohol as a form of self-medication.

"I didn't want anyone else to know, to share my problems with people, that I'm unhappy because I'm living what is, you know, a dream life for an Australian," he said.

"I am a tremendously successful athlete. I should be having the time of my life, and I'm not.

"And so I decided to keep it from people when I know that the best thing I could have done was to actually speak to my friends, to include my family in this, for people to watch out for me.

"But I wanted to, in some way, protect those people that were close to me... because I saw how happy they were from things that I did and I wanted them to have that."

Asked to describe what living with depression is like, Thorpe said: "You can't get out of bed. You don't want to get out of bed. You know, the only reason you will get out of bed is to use the bathroom."

Sir Michael told News Corp he had wanted to interview Thorpe for a long time.

"Ian Thorpe has always been near the top of my list to interview. The reasons are obvious. Not many athletes can claim to be the best of all time. Ian can," he said.

During the interview, Sir Michael also asked Thorpe how old he was when his father made a remark about his son's rising fame.

"He said 'I think I've just lost my son to the world'. I was 15 at the World Championships. Profound," Thorpe responded.

"It was accurate, it really was. My parents have wanted more than anything else to have their son back."

Australians rally around Thorpe's decision to come out

Australians have rallied behind Thorpe on social media, congratulating Thorpe for his decision to come out.

Actor Magda Szubanski tweeted her support saying: "So happy & proud for @IanThorpe Coming Out in his own time on his own terms. Wish you all things good darling boy. It's fun & free Out here!"

Fellow Olympic swimmer Grant Hackett said he hoped Thorpe's decision to speak publicly will help him move forward.

"Ian and I grew up together. Obviously we went through our careers together and we raced a lot ... but at the end of the day ... who really cares? It's 2014, I don't think anybody's really too concerned about his sexual orientation," Hackett said.

"He's been through a lot this year with depression and other things and hopefully that can allow him, as a person, to move forward."

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Australian Marriage Equality's Rodney Croome said the move would inspire tens of thousands of young gay people.

"They'll see that he himself has gone through a struggle and come out the other side a happier and more comfortable man and that he's being supported for what he's said," Mr Croome said.

"I think that will inspire other Australians, particularly young Australians, to come out as well. I think what Ian has done will save lives.

"Many gay people do experience anxiety and depression because of prejudice, ... discrimination and because they're treated unequally by the laws of the land including by the Marriage Act."

Thorpe later took to Twitter to thank people for their support.

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Interview comes as AFL commentator apologises for gay sledge

Thorpe's confession to Parkinson came as AFL commenter Brian Taylor was slammed for referring to a player as a "poofter" on live television.

Taylor apologised, but openly gay Australian Rules footballer Jason Ball told ABC radio that homophobic language has been harmful to him personally.

"From my own experience being gay and playing sport, homophobic language was a constant reminder that I wouldn't be accepted if I was to come out," he said.

"That language made me second guess everything that I ever said or did out of fear that someone would figure it out and I think Ian Thorpe has probably been in the same situation throughout most of his life."

Ball said Thorpe will be a role model for other athletes in the public eye who are afraid of coming out.

"Ian Thorpe was quite young when he came into the public spotlight and it's understandable from that early age when you're still figuring things out for yourself, that you can't be expected to come out publicly and be a role model," he said.

"This has no doubt been a source of struggle for Ian Thorpe over many years and I think that his battle with depression and anxiety are possibly linked.

"I offer a big congratulations to Ian Thorpe, I think it would be a big relief for him and no doubt he'll be a role model now for countless people out there who are struggling to come to terms with their sexuality."

The Gay and Lesbian International Sports Association (GLISA) also paid tribute to Thorpe, acknowledging the "psychological pain" he endured while keeping his sexuality a secret.

"For whatever his reasons, which must be respected, he felt unable to be open about his sexuality during the height of his sporting career, which has come at a huge personal cost," president Barry Taylor said in a statement.

"We salute Ian Thorpe for all he achieved in his swimming and we wish him well for the future with the hope that he finds a greater inner peace and a freedom and that he will continue to inspire people as he done as an athlete."

The Human Rights Commission's Susan Ryan - a former senator who penned the 1984 Sex Discrimination Act - said Thorpe's sexuality is his own business, but by speaking out he could help others struggling with their sexual identity.

"I think it just might help some of those teenage kids who think 'Oh, is it alright? Should I try not to be?' ... In the end, it's his business, but it could have a positive effect on young people who are having a bit of a struggle around this question of their sexual identity," she said.