The coalition Government today warned peers against falling for a Labour "elephant trap" which could scupper the timetable for its planned referendum on a new voting system.
Justice minister Lord McNally said a bid to be launched in the upper chamber this afternoon by ex-Lord Chancellor Lord Falconer to have the measure - which also cuts the number of MPs by 50 and redraws constituency boundaries - declared a hybrid Bill "doesn't have a leg to stand on".
If the Bill were declared hybrid - meaning it affects particular as well as general interests - it would be shunted off to a committee and the timetable for a poll on whether to adopt the new AV voting system could be thrown into chaos.
The Government wants it to take place next May, at the same time as a raft of elections across the UK.
Lord Falconer said today the AV vote could be taken out into a separate Bill to secure the timetable while the re-drawing of boundaries - which Labour opposes, as it would be expected to cut their MPs - is looked at separately.
He told BBC Radio 4's Today programme: "I'm not trying to scupper the Bill at all. There's nothing to stop the coalition, if they want a referendum, to take it out into a separate Bill."
But he argued the measure was hybrid because two constituencies, Shetland and the Western Isles, were singled out for special treatment.
And he said the move to reduce the number of MPs by 50 would weaken the Commons.
He said: "We need a strong House of Commons to hold the Government to account. If what you do is keep the Government the same size and at the same time take away 50 backbenchers, surely that's going to weaken the strength of the Commons."
Lord McNally told the programme: "I went to the Clerk of the House of Lords, who is the expert, and he says Charlie (Lord Falconer) doesn't have a leg to stand on."
He added: "If the House of Lords falls for this elephant trap then they are not as smart as I think their lordships are."
No party has a majority in the House of Lords and although Tories and Liberal Democrats combined could outvote Labour there are also large numbers of crossbench peers who could swing the vote.
The motion on whether to declare the Bill hybrid is expected to be voted on this afternoon.
The measure being discussed by peers equalises the size of constituencies at around 75,000 voters, a measure which is expected to hit Labour hardest.
It also reduces the number of MPs and allows the AV referendum to go ahead. The ballot on changing the voting system was a crucial part of the coalition agreement between the Conservatives and the Liberal Democrats.