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Replacing the system used to elect MPs will be "costly and complex", opponents of the change said as they launched their campaign.

The cost of staging a referendum and ditching first-past-the-post in favour of Alternative Vote (AV) would be £250m, the No to AV group claimed.

The Yes to Fairer Votes group disputed the figures and said their rivals were "desperate" and "short of arguments".

The row came as MPs resumed debating a bill allowing the referendum to happen.

The protracted battle to get parliamentary approval for the referendum to take place - which must happen by Wednesday if the poll is to be staged on 5 May - continued as MPs debated changes to the legislation proposed by the House of Lords.

'Lack of debate'

At its launch on Tuesday, the cross-party No campaign said spending money on changing the Westminster voting system would be wrong at a time when many people believe the country's focus should be on economic recovery.

It said the referendum would 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 worked in time for the next general election.

"The British people will be staggered to learn that our politicians are considering squandering £250m of taxpayers money on a new voting system at a time they are being told to tighten their belts," Matthew Elliott, director of the No campaign, said.

Speaking at the same event, fertility expert and Labour peer Lord Winston questioned claims that AV would be fairer than the current system and said the record of the coalition government was a sign of the "dangers" that could result from a system likely to produce Hung Parliaments on a regular basis.

"There has not been a proper debate and, in my view, there needs to be a proper consideration before we actually vote on a system which many of us will see as deeply flawed," he said.

'Fantasy'

The Yes campaign dismissed its opponents' claims as "fantasy", saying Australia had used the AV system for 80 years but had always counted ballots by hand and there were "no plans" to switch to electronic voting in the UK whatever the outcome of the referendum.

It also said its rivals had exaggerated the cost of voter education publicity and pointed out that the cost of the referendum - which Nick Clegg has said will be reduced by £17m by staging it on the same day as devolved elections in Scotland, Wales and Northern Ireland - would be incurred whatever the outcome.

"The No camp's sums, like their arguments, simply do not add up," Katie Ghose, chair of the Yes campaign, said.

"Having given up on defending first-past-the-post, the desperate No camp is descending into fantasy. Short on arguments, the Nos are trying to claim we cannot afford change. After the expenses crisis, we cannot afford not to."

Ministers said there were no plans to introduce electronic counting for future elections following a pilot study conducted between 2002 and 2007.

Parliamentary battle

The referendum campaign will only officially begin when the bill authorising the poll becomes law.

MPs will resume debating the legislation - 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 - put into the bill 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 Conservative 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.