Conversation so far:
What is peerage?
A peerage is a legal system historically comprising various hereditary titles.
How was peerage reformed by Tony Benn?
Benn continued his campaign outside Parliament. The Peerage Act 1963, allowing lifetime disclaimer of peerages, became law shortly after 6 pm on 31 July 1963.
What was disclaimer of peerages in the UK?
To disclaim a hereditary peerage, the peer must deliver an instrument of disclaimer to the Lord Chancellor.
Why did Tony Benn renounce his title?
Benn automatically became a peer, preventing him from sitting in the House of Commons.

Question: Why did he automatically become a peer?
Self-contained rewrite: Why did Tony Benn automatically become a peer?