The poll is due to be held on 5 May Replacing the system used to elect MPs will be “costly and complex”, opponents of change will say as they launch their campaign for the 5 May referendum.
The cost of staging the poll and ditching first-past-the-post in favour of Alternative Vote (AV) will be £250m, the No to AV campaign will claim.
It will argue the issue is “obscure” and a low priority for the public.
The Yes to Fairer Votes group said it was “gaining momentum” and named actor Colin Firth among its supporters.
The developments come as the protracted battle to get Parliamentary approval for the referendum to take place in May comes to a head.
At its launch on Tuesday, the cross-party No campaign is expected to spell out what it says will be the cost of changing the Westminster voting system at a time when it believes many people believe the country’s focus should be on economic recovery rather than constitutional change.
It will say the referendum will cost about £90m and that, should voters back a switch to the AV system, £130m would have to be spent on electronic vote counting machines and £26m on informing the public how the new system works in time for the next general election.
“We have got a very clear message”
Matthew Elliott Director, No to AV campaign
Setting out the arguments for a No vote, it is likely to question claims that AV is fairer than the current system and highlight cases of “broken promises” by the coalition which it says could be a harbinger of what will happen if AV forces parties to share power on a regular basis.
“We have got a very clear message. A move towards AV would be costly to the taxpayer, would also bring greater complexity to our electoral system and produce less accountable government,” campaign director Matthew Elliott told the BBC ahead of the launch.
He doubted whether the public are interested in voting change: “Because at the moment this is such an obscure issue for people, it is not very high on their list of priorities.”
Deputy Prime Minister Nick Clegg, who supports a switch to AV, has said staging the poll on the same day as devolved elections in Scotland, Wales and Northern Ireland will save the taxpayer about £17m.
Both sides will also unveil a number of new supporters on Tuesday as the campaign steps up.
The Yes campaign will say it has been endorsed by the comedian Eddie Izzard and the former BBC director general Greg Dyke as well as actors Helena Bonham-Carter and Colin Firth.
“The referendum is a once in a generation opportunity to change our clapped out politics for good,” Mr Firth, Bafta winning star of the film The King’s Speech, said in a statement.
The actor had been a prominent supporter of the Lib Dems – which are campaigning in favour of AV – but said recently that the coalition’s spending cuts meant he could no longer give them his backing.
The Yes campaign argues AV will help boost trust in Parliament by requiring candidates to get more than 50% of votes cast before being elected and forcing them to appeal to a broader base of support.
Its chair Katie Ghose said: “There is a mood for change in the way we do politics in this country. People want their MPs to have to work harder and be more responsive to their real concerns.”
The referendum campaign will only officially begin when the bill authorising the poll becomes law – which is expected to happen on Thursday.
Under the AV system, voters rank candidates in their constituency in order of preference.
Anyone getting more than 50% of first-preference votes is elected.
If no-one gets 50% of votes, the candidate with the fewest votes is eliminated and their backers’ second choices allocated to those remaining.
This process continues until one candidate has at least 50% of all votes in that round.
Vote change: Where parties stand Q&A: Alternative Vote referendum
MPs will resume debating the bill – which also includes measures to redraw parliamentary boundaries and cut the number of MPs by 50 – on Tuesday after it was finally approved by the House of Lords following weeks of lengthy and often acrimonious debate.
Among issues to be considered by MPs is a proposal – backed recently by peers – for the referendum to be legally binding only if 40% of the electorate take part.
Prime Minister David Cameron is due to set out his case for retaining the existing system – a position backed by most of his MPs – in a speech on Friday. Labour leader Ed Miliband supports a switch to AV although opinion within his party is divided.
Under first-past-the-post, voters are allowed to vote for one candidate and the individual obtaining the most votes is elected.
Under the AV system, voters rank candidates in their constituency in order of preference.
Anyone getting more than 50% of first-preference votes is elected and if no-one gets 50% of votes, the candidate with the fewest votes is eliminated and their backers’ second choices allocated to those remaining. This process continues until one candidate has at least 50% of all votes in that round.
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