The Government is expected to set out plans within a month to reform the House of Lords, which will still leave space for peers nominated by political leaders.
The civil rights lawyer Clive Stafford Smith said that preparations were underway to mount a legal action against any proposal for a second chamber that included unelected members.
Delivering the annual Longford Lecture, sponsored by The Independent, he argued the principle of one-person one-vote had to be sacrosanct. Ridiculing the concept of laws being made in the second chamber by a "bunch of Tony's cronies", he said: "I plan to be part of the legal team that pursues these people and gets this idea struck down. The mother of Western democracy cannot lag behind in its evolution."
It is understood the challenge would be mounted under provisions of the European Convention guaranteeing citizens a free choice in the composition of their legislature.
Mr Stafford Smith is British-born but has represented death row prisoners in the United States as well as Guantanamo Bay detainees.