Pressure is mounting for a full TV debate ahead of the election
Gordon Brown, David Cameron and Nick Clegg are to sit side-by-side and be grilled by MPs live on television.
They will discuss the best way to get more women, ethnic minority and disabled people to enter Parliament.
The hearing next week comes as the leaders are being urged to hold US-style TV debates ahead of the election.
Lib Dem Mr Clegg and Conservative Mr Cameron have agreed to take part, while Labour's Mr Brown says he is willing to do so "in principle".
The opposition party leaders regularly trade verbal blows one-on-one with Mr Brown at prime minister's questions.
However, their appearance side-by-side as part of the Speaker's Conference - is thought to be unprecedented.
The proceedings will be broadcast live. A spokesman said: "The leaders will be sitting side-by-side, in the same way as they would when facing a parliamentary select committee.
"The MPs will ask questions of one, two or all the leaders, as with select committees."
The conference, chaired by Commons Speaker John Bercow, was set up by his predecessor Michael Martin.
Parliament is also planning to run a web forum during the question-and-answer session next Tuesday. You will be able to watch the whole event live on BBC Parliament and the BBC News website from 1030 BST on Tuesday 20 October