Image caption MPs have yet to vote on the plan for a referendum

The campaign to ditch Britain's first-past-the-post electoral system in favour of the Alternative Vote will be "fun," its organisers have promised.

James Graham, of Unlock Democracy, ruled out "angry" protests and "dirty tricks" in the push for a change to AV.

The pro-AV campaign, backed by several electoral reform groups, is expected to be launched within the next fortnight.

The anti-AV campaign, which is already up and running, said it also wanted an "open and honest debate".

A referendum will take place on 5 May next year, if Parliament backs the idea when it returns next month.

Electoral reform has traditionally been seen as an arcane subject only of interest to a handful of academics and campaigners and both sides face the challenge of bringing the issue to life for a general audience.

'Not protests'

A number of organisations, including Unlock Democracy and the Electoral Reform Society, have been campaigning for years to scrap Britain's first-past-the-post electoral system but they will be coming together to form a new body to campaign for a change to AV.

This referendum is above party politics Matthew Elliott, Anti-AV campaign leader

Mr Graham denied they will be in danger of tripping over each other, saying they had been meeting regularly since the election and "so far it has been quite coherent".

He said the Yes to AV campaign was also keen to counter the impression that it will be a Liberal Democrat "front" organisation - stressing it will be a cross-party, non-partisan organisation and not run out of the Lib Dems' Cowley Street headquarters as some bloggers have claimed.

The Lib Dems insisted on electoral reform as the price of forming a coalition with the Conservatives, even though most Tories, including party leader David Cameron, oppose it.

Mr Graham said his campaign would not be "Lib Dem-led", adding: "We are quite conscious that the campaign isn't perceived to be a Lib Dem campaign. There is an attempt to tar it as that."

He added that the pro-AV campaign, which is expected to hold an official launch on the weekend of 4 September, would not play "dirty".

But it wanted to tap into the momentum generated by the Fair Votes Now campaigners who marched on Westminster during in May, to put pressure on Lib Dem leader Nick Clegg not to "sell out" on electoral reform during coalition negotiations.

He said Unlock Democracy had been "surprised" by the scale of the marches, adding: "it is not every day you see people marching for electoral reform".

Business for Sterling

He suggested there could be similar events once the AV campaign begins, as well as a series of music events across the country. Efforts were also under way to recruit celebrity supporters, he said, stressing that the campaign wanted to come across as "fun rather than angry".

Both sides in the referendum debate are limited to spending £5m under Electoral Commission rules, with each entitled to up to £600,000 from the taxpayer.

They will be allowed to send one leaflet to each UK voter at the taxpayers' expense. The Electoral Commission is also expected to mount a public information campaign on AV, although the details have yet to be confirmed.

The anti-AV group - called No2AV - has stolen an early march on its rivals by appointing Matthew Elliott, chief executive of the Taxpayers' Alliance, to run its campaign, widely seen as one of the most effective lobbyists at Westminster.

Conservative peer Lord Leach, who funded the anti-euro Business for Sterling pressure group, and Conservative MP George Eustice, a leading figure in that campaign, are expected to play prominent roles.

But Mr Elliott denied claims by Mr Graham that he is a "partisan" figure.

'Non-partisan campaigner'

He told BBC News: "We will be fighting our campaign in the spirit of open and honest debate, because we want to see more referendums in the future. I am pleased to hear the Yes campaign are looking to do similar.

"This referendum is above party politics. The issue is about the democracy of our country and I'm confident that those who agree AV would be wrong for Britain will be able to put aside short-term party-political issues.

"There will be lots of Conservatives supporting the campaign, just as there will be many Labour and Lib Dem supporters. As a non-partisan campaigner, I look forward to bringing them together."

AV, which would replace the current first-past-the-post system for Westminster elections, allows voters to rank candidates in their constituency in order of preference.

Anyone getting more than 50% of first-choice votes in the first round is elected, otherwise the candidate with the fewest votes is eliminated and their backers' second choices allocated to those remaining. This process continues until a winner emerges.

The only previous UK-wide referendum was in 1975, when the public voted in favour of staying in the European Community, which later became the European Union.