By politics.co.uk staff
David Cameron's knows AV is wrong in his "guts", he told voters on a day that saw the debate on electoral reform cross party lines.
The prime minister was speaking at a No to AV event in London opposite John Reid, former Labour home secretary.
"It is obscure, unfair and expensive," Mr Cameron said of the AV system.
"It will mean that people who come third in elections can end up winning. It will make our politics less accountable and it would be a backward step for our country.
"Too often debates about AV are less like political arguments, and more like scientific discussions, where people get lost in a language of proportionality and preferences, probabilities and possibilities.
"Of course, some of these things are important. But for me, politics shouldn't be some mind-bending exercise. It's about what you feel in your gut - about the values you hold dear and the beliefs you instinctively have. And I just feel it, in my gut, that AV is wrong."
Mr Cameron argued that AV reduces voter power and politicians' accountability.
"I believe in the principle of one person, one vote. But AV will mean the votes of some people get counted more than others," Mr Cameron said.
"The biggest danger right now is that Britain sleepwalks into this second-rate system, waking up on May 6th with a voting system that damages our democracy permanently. We must not let that happen."
Lord Reid - a stringent critic of the Tories, often labelled as a Labour 'attack dog' - said the vote on AV "transcended party politics".
"David and I are party people," he said. "But a political party is not an end in itself."
"There are some issues so important that they transcend party politics. Where we should rise above personal differences," he said.
"This is a positive campaign. It is defending the right of one person, one vote - a system that has been the cornerstone of our democracy for generations. That is the British way, it is the fairest way and it is the best way."
The former home secretary directed his bite towards the Liberal Democrats.
"It would not only be wrong, it would be an outrage to try to secure a change to the electoral system for tactical party advantage," he added.
"But there is a growing and well-founded suspicion that is at least partly the aims of the 'yes' campaign, and I include in that the leadership of the Lib Dems. Not a change in the public interest but a change, to some extent at least, based on narrow self-interest."