David Cameron issued a warning to voters yesterday not to “sleepwalk” into a change of voting system for Westminster elections with damaging consequences for Britain’s democracy.

The Prime Minister warned that with millions of voters so far showing little interest in the May 5 referendum on the adoption of the alternative vote (AV) for parliamentary elections, the Yes campaign could triumph by default.

As the two main party leaders fronted rival No and Yes campaign events, Labour’s Ed Miliband accused the No campaign of spreading “groundless fears” that AV would help extremist parties like the British National Party.

Senior Tories, meanwhile, signalled they could oppose legislation to introduce AV based on a strong Yes vote in Scotland, Wales and Northern Ireland, where turnout is likely to be boosted by parliamentary and assembly elections on the same day.

Mr Cameron, appearing on a cross-party No to AV platform with the Labour former home secretary Lord Reid, warned that a low turnout overall could favour the Yes campaign.

He described the unwillingness of many voters to engage with the arguments over AV as a “problem”.

“The biggest danger right now is that Britain sleepwalks into this second-rate system that damages our democracy,” he said.

The Prime Minister said of the AV system that “it’s obscure, it’s unfair, it’s expensive” and that it could mean “people who come third in elections will end up winning”. “I think that is unfair and wrong, and flies in the face of centuries of our history,” he said.

Mr Cameron sought to distance himself from the No campaign attacks on Deputy Prime Minister Nick Clegg, whose Liberal Democrat party strongly supports voting reform.

“I don’t run the No campaign, I run the Conservative No campaign,” he said. “I certainly don’t condone any personal attacks on anyone in this campaign.”

However, Lord Reid – standing alongside the Prime Minister – accused the Lib Dems of pushing for AV out of “narrow self-interest” because they believed it would help them in future parliamentary elections.

“It would not only be wrong but it would be an outrage to try and secure a change in the electoral system for tactical party advantage by usurping the right of all of our citizens to an equal vote,” he said.

“There is, I have to say, a growing and well-founded suspicion that that is exactly at least part of the aims of the Yes campaign and I include in that the leadership of the Lib Dems.” Lord Reid also warned of the dangers if the Government tried to push through a major constitutional change after “a very, very low turnout” in the referendum.

His concerns were echoed by senior Tory backbencher Eleanor Laing, who warned the result would be “called into question” if there was a Yes vote as a result of a disproportionate level of support for AV in Scotland and Wales.

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