London Mayor Boris Johnson has called for the House of Lords to be halved in size to 400 peers.
Mr Johnson, who is a Conservative MP and frequently tipped as a potential successor to David Cameron as leader, said that peers who did not contribute to the House's deliberations should be made "an offer they can't refuse" to leave.
Controversy over the size of the second chamber was revived last week by Mr Cameron's appointment of 45 new peers, including 26 Tories, bringing total numbers to more than 800. The House was already the world's second largest legislative assembly.
Speaking on LBC radio, Mr Johnson said there was a need for "pruning" in the upper House: "I think the whole thing has got completely out of control ... Something radical needs to happen there."
He added: "There's a great many of these geezers, who don't actually do anything very much at all. They take the money, and they don't do much. And it seems to me, we should make them an offer they can't refuse.
"If they don't turn up, if they're not really interested, they're just interested in the glory of being a member of the House of Lords - stuff that, that's absolutely absurd. We probably need about 400 legislators to scrutinise bills, to amend them, and to improve them."
Mr Johnson said it was "absurd" that Mr Cameron was forced to nominate large numbers of new Conservative peers to counter "a pretty big anti-Tory majority" in the Lords.
"The Government has got to be able to get its business through. It would be completely undemocratic if the unelected House of Lords were able to block it," he said.
"I think it's really the only option the Prime Minister has. I think he's got... in the current circumstances, he's doing the only thing he really can do. And I totally defend and support him on that, but this thing needs reform."