Cameron and Clegg in Leeds clash over voting reform

The Prime Minister warned that introducing the alternative vote system would be “a massive backward step for accountability and trust in our politics”.

But his Liberal Democrat deputy insisted it was the only way to tackle corruption at Westminster and prevent millions of voters being “ignored”.

The coalition allies used speeches delivered hours apart to fire their first salvoes in the campaign.

Both men stressed that the result of the ballot on May 5 was not make-or-break for the Government.

However, there were early signs of the potential tensions when Mr Cameron took Mr Clegg to task for supporting AV after previously branding it a “miserable little compromise”.

Despite senior Tories and Lib Dems extolling the virtues of coalition over the past nine months, the premier also expressed concern that hung parliaments would become “commonplace”.

“It won’t surprise you to hear me say that is not necessarily a bad thing and that, as happened last May, it can bring parties together in the national interest,” he told an audience in central London.

“But let’s be clear, when there are more hung parliaments there will be more haggling and horsetrading between politicians - both before and after elections.

“There will be gamesmanship between parties in different constituencies as they try to stitch up second preference votes.

“And there could well be an occasion where we have a genuine second-choice government.

“If the last election was under AV, there would be the chance, right now, that Gordon Brown would still be prime minister.”

He said although first past the post had not delivered a decisive verdict at the last election “in terms of who won”, it was “decisive in terms of who lost”.

“I think any system that keeps dead governments living on life support is a massive backward step for accountability and trust in our politics,” Mr Cameron added.

Speaking in Leeds earlier, Mr Clegg blamed the existing voting system for encouraging politicians to abuse expenses.

“For years, politicians and parties have courted the votes of a few thousand people in marginal seats and ignored the rest,” he said.

“It is because there are so many MPs with jobs for life that there are so many who can take their constituents for granted.

“And it is because there were so many MPs taking their constituents for granted that so many abused their expenses.

“There was a clear link between how safe an MP’s seat was and how likely they were to abuse the system.

“When a person is corrupt, they should be punished.

“When a system makes corruption more likely, it should be changed.”

He urged the public to “remember how it felt when you heard about MPs spending your money on duck houses and having their moat cleaned”, insisting: “We deserve something different.”

While first past the post was “outdated”, AV would make MPs “work harder”, according to Mr Clegg.

“It means that parties will have to compete for votes in every corner of the country and not just those few marginal seats,” he said.

“It means more people get listened to and more respect for the different opinions and feelings we share as a nation.”

Mr Clegg dismissed objections that AV was too complicated, pointing out that the system of ranking candidates by preference was already used in Australia and London mayoral elections.

In an apparent nod to Lib Dem supporters - who have long campaigned for a more fundamental shift to proportional representation - the Deputy Prime Minister conceded that AV was “evolution not revolution”.

“It’s a small change which will make a big difference,” he said.

Despite the claims of critics, the proposed system would also produce “strong, stable governments”, according to Mr Clegg.

“The only election that would have resulted in a hung parliament was last year’s, just as it did under first past the post,” he said.

But Mr Cameron attacked many of his erstwhile colleague’s arguments head-on, stating bluntly that they “profoundly” disagreed.

He described the idea that AV was more fair and proportional as “a myth”, insisting it would mean some people’s votes counted more than others’.

“If you vote for a mainstream candidate who is top of the ballot in the first round, your other preferences will never be counted,” Mr Cameron said.

“But if you vote for a fringe party who gets knocked out, your other preferences will be counted.

“In other words, you get another bite of the cherry.

“I don’t see why voters of the BNP or Monster Raving Loony Party should get their votes counted more times than supporters of the Conservatives or, for that matter, Labour or Liberal Democrats.”

Rather than delivering victory for the candidate with the most positive backing, AV would reflect “passive acceptance” and result in a “parliament of second choices”.

“It can mean someone who’s not really wanted by anyone winning an election because they were the least unliked,” Mr Cameron said.

“It could mean that those who are courageous and brave and may not believe in or say things that everyone agrees with are pushed out of politics and those who are boring and the least controversial limping to victory.”

The Prime Minister said there was a “brilliant simplicity” to first past the post, while AV was so complex that a “whole machinery of bureaucracy will have to be built to explain the system to people”.

He claimed most of AV’s backers really wanted a proportional representation system - pointing out that last April Mr Clegg had dismissed AV as a “miserable little compromise”.

However, the Tory leader insisted the referendum campaign would not be a “source of tension” with his Lib Dem deputy or a “coalition breaker”.

And Mr Clegg said: “Whatever the result, we will continue to work together in the national interest.”

Answering questions after his speech, Mr Cameron said he would have preferred to hold the referendum later in the Parliament.

However, he decided to accept the date because there were “things I needed to achieve through the coalition”.

“It’s absolutely no secret that I did not want the referendum to take place on May 5 - I was quite keen for it to happen later in the Parliament,” Mr Cameron said.

“This was extremely important to the Liberal Democrats.

“There are things I need to achieve through the coalition and in the end I decided it was right to have this referendum on May 5.

“If there is a No vote, the Liberal Democrats are able to say to their supporters ‘We had the referendum we got in the coalition agreement and we had it on the day of our choosing’.”