The Speaker of the House of Lords, Lord Fowler, says he is standing down to "speak his mind" on issues he cares about like Aids.
The peer, who chairs debates in the Lords and who has been in post since 2016, said while he had independence, he didn't have the freedom to campaign.
He said that at the age of 83, "unless he was careful" he wouldn't be able to "start a new career".
He will leave his post in April, a few months earlier than planned.
Lord Fowler said he had been in Parliament for over half a century but had never been a truly independent member of the House.
As secretary of state for health in the 1980s, he was in charge of the government's response to the Aids epidemic.
He said speaking out on this issue was one of the reasons for his early departure.
He added: "The position of Lord Speaker suffers from one grave disadvantage: I have independence but, entirely properly, I am not able to speak my mind on issues of the day."
He said he wanted to start a new career as an "entirely independent backbencher able to speak out on issues that concern me like the size of the House and to have the freedom to campaign in the area of HIV and Aids".
Lord Fowler added that 35 million people around the world had died as a result of the Aids pandemic and there were "examples beyond count" of persecution against LGBT people worldwide.
He said he would spend the next years campaigning against these "modern evils" and support individuals in the field working to "turn the tide".
The Speaker of the House of Commons, Sir Lindsay Hoyle, paid tribute to Lord Fowler.
He said he should be congratulated for overseeing the creation of a "very successful" working model for the House of Lords during the pandemic.
He added: "His decision to step down as Lord Speaker to continue his relentless campaign for awareness of HIV and Aids is commendable.
"I am in no doubt that his 'Don't Die of Ignorance' awareness campaign when he was Health Secretary was a life saver, in that it showed his grasp of the enormity of the Aids epidemic at a time when gay sex was a taboo topic."