The former Labour culture secretary, Chris Smith, has revealed he has been HIV positive for 17 years.
Chris Smith says Nelson Mandela inspired him to go public
Mr Smith, 53, told the Sunday Times he did not tell Tony Blair about his illness when he became Britain's first openly gay Cabinet minister in 1997.
He said he decided to go public about his condition after former South African president Nelson Mandela announced his son had died of Aids.
Mr Smith was diagnosed in 1987, but did not know when he contracted the virus.
He told GMTV's The Sunday Programme he had previously worried about the impact on his parents and others.
"Over recent weeks and months I have thought perhaps this is something that needs to be spoken about," he said.
Mr Smith, who is due to stand down at the next election, said he wanted to "demystify" the condition in the public's perception and to challenge prejudices.
He said HIV was a "liveable and treatable condition".
"It means you have to take quite a lot of pills, but if you carry on doing that, there's no reason why you should not be able to go on living with this condition."
But he admitted in the Sunday Times he had been "really worried" when he first found out, because of the lack of treatment around at the time.
His decision to go public over his condition was praised by campaigners.
Lisa Power, head of policy for the HIV charity Terrence Higgins Trust, told BBC News it was "extremely important" that he has spoken out now.
"Hopefully this will help a lot of people who haven't been diagnosed yet and those that are too afraid to have tests," she said.
"There is still a great deal of prejudice and ignorance surrounding HIV. Many people with the condition are still too scared to come out and often they won't tell their employers or their fellow employees because they are scared of what they may think.
"But it is possible to live with HIV and you can lead a normal life."
Peter Tatchell, of gay rights' group OutRage! supported Mr Smith's move but said he was surprised it had taken him so long, especially as Mr Smith's coming out as gay had not hurt his political career.
"I hope other prominent public figures with HIV will follow Chris's lead," he said.
"The more people who are out about being HIV-positive, the sooner we will help break down ignorance and stigma."
Mr Smith, who revealed that he was gay in 1984, a year after becoming MP for Islington South, said he had not told Mr Blair about his condition in 1997 because he felt his performance would not be affected.
Mr Smith was sacked in the 2001 reshuffle
He told the Sunday Times: "I didn't feel the need to tell people, except for a very, very few, as it was not in any way affecting my work."
Mr Smith said Mr Mandela's call for an end to the stigma surrounding Aids, made after the death of his 54-year-old son Makgatho, had prompted him to speak out.
"What Nelson Mandela said very much struck a chord with me," he said.
The MP, who has always been treated by the NHS, said he was lucky to soon be put on a combination of anti-retroviral drugs that have helped him fight the illness.
He said a sensible diet had also helped him maintain his health.
Mr Smith was sacked from the Cabinet after the 2001 election, following controversies over the Wembley stadium project and the lottery.
In 2003 he announced he would quit politics at the end of the current parliament.