After years of waiting, reform campaign reaches for its place in history

Campaigning for the historic referendum on voting reform burst into life yesterday as Ed Miliband joined Liberal Democrats and Greens in a cross-party push for a Yes vote.

The Labour leader, whose party is divided on the issue, issued a powerful plea for changing from the existing first-past-the-post system to the alternative vote (AV). He said: "The case for AV is fundamentally a case for political change. If people are happy with politics as usual then they should vote No. But if not, they should vote Yes because AV is a step in the right direction – not a panacea – for changing politics as usual."

Mr Miliband argued that AV would help progressive centre-left parties to "build bridges not barriers". He said: "For most of the last 80 years, there has been one Conservative Party but several competing for progressive votes. No wonder the Tories back the current system. They know Britain is not a fundamentally Conservative country. But with first past the post, they are more likely to govern whenever progressive forces are divided."

Mr Miliband shared a platform at the Methodist Central Hall, Westminster, with the Liberal Democrats Baroness Williams of Crosby, Charles Kennedy, and party president Tim Farron. Green Party leader Caroline Lucas and London Assembly member Darren Johnson were also there alongside the Shadow Cabinet members Tessa Jowell and John Denham.

But the rival No camp drew attention to the man missing from the stage: Nick Clegg. The Liberal Democrat leader is in Mexico on an official visit but Mr Miliband had refused to appear alongside him, fearing his unpopularity would lose the Yes campaign votes. The formal launch of the Yes campaign for the 5 May referendum will take place on Saturday.

Last night, a row broke out over the funding of the rival campaigns. Although they are both allowed by law to spend £5m, the Yes lobby claimed it would be outgunned because the Conservative Party would spend millions on top of that – including material opposing AV in its campaigns for the Scottish Parliament, Welsh Assembly and English council elections on the same day as the referendum. Lord Oakeshott, a Liberal Democrat peer, challenged David Cameron in a letter to come clean about the Tories' spending on the referendum, describing it as "the campaign in dark glasses".

Tory officials dismissed claims £4m has been allocated to the referendum as "a laughably wild exaggeration". They admitted that staff from Conservative Campaign Headquarters had been seconded to the No camp, which said all its backers would be named shortly. It pointed out that Liberal Democrat candidates were including pro-AV messages in their election material. Matthew Elliott, campaign director of No to AV, dismissed yesterday's cross-party event as "a Lib Dem convention chaired by Ed Miliband".

He said: "The only person missing from their Westminster photo-op is the man who is forcing us to have this expensive referendum: Nick Clegg."

Jane Kennedy, a former Labour minister who opposes electoral reform, said Mr Miliband's claim that AV would boost progressive politics was untrue. "It is disappointing that the Yes campaign is trying to seize the progressive tag to further the cause for a voting system that is complex, costly and unfair. It is even more disappointing that the leader of the Labour Party is willing to talk up an electoral pact with the Lib Dems who have so betrayed Britain," she said.

Charles Kennedy said it was vital that the supporters of electoral reform took the opportunity presented by the referendum. "This represents the force of political reform," he said. "This is a chance that has got to be seized."

Tim Farron admitted his irritation at the dispute over whether Labour was prepared to campaign alongside Mr Clegg. "I think it would be wrong for us to get into silly rows about who should or should not be on a platform," he said.

Independent values

In the aftermath of the 2005 general election, The Independent launched a campaign to overhaul Britain's antiquated voting system. What began as a front-page campaign quickly snowballed into a clamouring demand for change. Within a month, 40,000 people had signed our petition and hundreds of MPs from across party lines had also come out in favour of electoral reform.By the time the 2010 election had come around, all three major parties had included various electoral reform promises in their manifestos. The campaign to overhaul our unrepresentative voting system begins anew, and it must be won.

Process: How would the alternative vote work in practice?

* The main difference between the voting system known as the alternative vote (AV) and the one we now use, known as first past the post, is that instead of putting a cross against one candidate only, under AV you can vote for as many as you like, but have to number them in order of preference.

* Once the polls have closed, the first thing the Returning Officer's staff will do is count the first preference votes. If one candidate has secured more than 50 per cent, counting ends there, and that candidate is declared the winner.

* But in many seats, there will be no outright winner. In that case, the candidate who came last is eliminated, and his or her second preference votes are re-allocated. If that does not produce an outright winner, another candidate is eliminated, and so on until someone has over 50 per cent of the vote.

* Though you are entitled to vote for more than one candidate under AV, you do not have to. If the candidate you support is likely to win or come second, there may actually be no point in voting for anyone else, because your second preference vote is unlikely to be counted. Opponents of AV object that is unfair, because the effect is that some people have more votes than others.

* AV is sometimes wrongly described as a form of proportional representation (PR). It is not. It does not guarantee that the number of MPs elected from any party will be proportionate to the number of votes cast for that party. Indeed, it has been calculated that if AV had applied while Tony Blair was leading the Labour Party he would have won three elections with even bigger majorities. The same applied to Margaret Thatcher in the 1980s. What AV would do is eliminate the phenomenon that commonly occurs under the present voting system under which an MP is elected with the support of less than half the electorate.

* Though AV does not wipe out the possibility of one party winning an outright majority, as PR would, many seats that are now so safe that the parties can afford to ignore them will become electoral battle grounds, and hung parliaments and coalition governments more likely.

* Though no one knows what the effect would be, it is reasonable to assume that there would be more Liberal Democrat MPs under AV, and fewer Conservatives, which is why the two parties are split over this. The effect on Labour's showing is harder to gauge. The smaller parties could expect to get more votes, but that might not result in their getting any more MPs.