A switch to the Alternative Vote for Westminster elections would give more power to fascists, Conservative co-chairman Baroness Warsi warned today.
Lady Warsi said that AV represented "a serious danger to our democracy" and urged voters to reject it in the nationwide referendum on May 5.
Ditching first-past-the-post for the AV system, under which voters rank candidates in order of preference and votes are redistributed as the least popular are eliminated, would "reward extremism and give oxygen to extremist groups" like the British National Party, she said.
But supporters of AV dismissed her claims, arguing that it was be "the most extremist-proof of all electoral systems" as it requires the eventual winner to secure the support of 50% of voters.
The Muslim Council of Britain and the Operation Black Vote campaign pointed out that the BNP are campaigning for a No vote, and argued that AV will force candidates to "reach out" to a broader constituency of voters.
Denouncing AV as a "disastrous, discredited and unfair voting system", Lady Warsi said it would encourage mainstream candidates to pander to supporters of fringe parties in the hope of benefiting from their second preferences once their first choice is eliminated.
Voters would feel free to give their first preference to extremists as a protest, secure in the knowledge that it would be transferred to a mainstream candidate in later rounds, giving parties like the BNP "more votes and more long-term legitimacy".
And with hung parliaments more likely under AV, smaller groups would often hold the balance of power and enjoy "greater stature and credibility" as they were courted by the leaders of larger parties seeking to form a coalition.
Lady Warsi warned that many supporters of AV saw it as "a stepping stone" to full proportional representation - a system which delivered the BNP two MEPs in the 2009 European elections.
Lady Warsi also said that AV would give "a second or third bite at the cherry" to backers of unpopular fringe candidates.
"It's not just the sheer unfairness of this which gets me," she said.
"It's the fact that for some completely arbitrary reason, AV gives more power to those people - fringe voters, Monster Raving Loonies, and yes, fascists - who are voting for precisely the kind of extreme policies most people want to marginalise.
"You don't need me to tell you that this represents a serious danger to our democracy.
"It means that bigots will be given more power in our politics and extremists will look to gain more influence over mainstream parties.
"The danger is that under AV, our whole political system would take a giant leap backwards, becoming more warped and disproportionate as fringe voters hold sway."
But Katie Ghose, chair of Yes to Fairer Votes, said: "The No campaign can't choose their supporters, but they can't escape the fact the BNP are campaigning for a No vote. Maybe up is down and black is white, but Nick Griffin is still saying No to AV."
Farooq Murad, secretary general of the Muslim Council of Britain said: "We believe that politics can be better.
"AV means that all voters will have a stronger say in our elections, and that all politicians will have to reach out further - and secure majority support from the communities they seek to represent.
"The BNP are campaigning for a No vote because they know what a Yes vote means - that racists who won't reach out have no future."
Simon Woolley from Operation Black Vote said: "AV means all MPs will have to reach out beyond a narrow targeted group of voters and represent people who often feel neglected by our present system.
"The fact that the BNP desperately want a No vote speaks volumes."
And Brij Mohan Gupta, chair of the Hindu Culture and Heritage Centre said: "AV will give more power to ordinary voters to decide the fate of the nation."
Liberal Democrat peer Baroness Falkner said she was "shocked and frankly appalled by the distortions being spun" by Baroness Warsi and the No campaign.
"While the BNP agree with Baroness Warsi and are calling for a 'No' vote, the Muslim Council of Britain and Operation Black Vote have strongly come out in favour of reform because to truly stop extremist parties getting elected, we need to vote Yes," said the Muslim peer.
"Under AV, no one can get elected unless the majority of people support them, which quite obviously makes it harder, not easier, for extremist parties. That's exactly why the BNP are campaigning for a 'No' vote.
"The No campaign has resorted to baseless scaremongering because they can't make any positive case for the status quo. People won't be fooled by this."
But Labour MP Keith Vaz said: "Many of the people campaigning for AV know it's a hopeless system. They readily admit what they really want is proportional representation. For them, AV is a stepping stone to that goal.
"So, what they're really telling us is they want to bring in a voting system that would guarantee the BNP seats in the House of Commons.
"The Yes campaign's ultimate goal is a system that lets the BNP in. I'm happy to stick with a system that keeps them out, that's why I'm saying No to AV."