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Opponents of changing the UK voting system are to launch a big advertising campaign shortly amid claims of "scare stories" tactics by their rivals.

The No to the Alternative Vote (AV) campaign is to argue money spent on the 5 May referendum could be used instead on health, education and defence.

But Yes to Fairer Votes said a similar current ad, using the image of an ill baby, was "shameful" and a "smear".

The elections watchdog said it issued guidance but could not regulate ads.

The row is the latest clash between the two official campaigns, which are leading the debate over whether the UK should keep its existing first-past-the-post system for electing MPs or switch to AV, in which voters rank candidates in order of preference.

The Yes campaign have taken issue with an advert in the placed by their opponents.

The ad, one of a series appearing in regional newspapers in recent days as the campaign gathers pace, juxtaposed a picture of a sick baby with the words "she needs a new cardiac facility not an alternative voting system".

Birmingham Children's Hospital wants to build a new £12m cardiac unit and launched a £2m fundraising drive last year.

The Yes campaign said the poster was "shocking and shameful" and claimed their opponents had "been reduced to dirty scare stories and smears to try and score a cheap political point".

"This is not the debate that the country deserves. It is clear that the No campaign does not have the decency and integrity to regulate themselves but this cannot go on," their chief executive Katie Ghose said.

The Yes campaign have urged the Advertising Standards Authority to "step in" and "issue guidance" on what advertising is acceptable in the two-month campaign, the first UK-wide referendum for 35 years.

"UK voters are entitled to legal, decent, honest and truthful advertising campaigns - these adverts fail on every count," Ms Ghose said.

Opponents of AV have been focusing their recent attacks on the cost of staging the referendum and of potentially switching to a different system, which they claim will cost the UK a total of £250m.

Their rivals have accused them of peddling "lies" about this, saying claims that £130m will have to be spent on electronic counting machines in the event of AV being adopted are false as ministers have confirmed they have no plans to introduce such procedures in future whatever the outcome of the referendum.

The Electoral Commission confirmed it had received a complaint about the Birmingham Mail ad but said it had no powers to regulate political advertising as it merely issued guidance on rules for individuals and groups taking part in election and referendum campaigns.

The Advertising Standards Authority, the self-regulatory body for TV, print and internet advertising in the UK, stopped regulating political advertising in 1997, after it got caught up in a row over a Conservative poster depicting Tony Blair with "demon eyes".

Legislation passed in 2000 provides scope for people to be prosecuted for statements or advertising deemed defamatory or which could incite racial hatred. But critics claim a loophole in the law allows misleading or factually incorrect material to go unchallenged.

The No campaign said they were entitled to raise the issue of the cost of the poll - estimated to be £82m - and any switch to AV. They confirmed they would launch a similar ad campaign in national newspapers early next month arguing the UK "cannot afford" a switch to AV.

One advert will feature an image of a soldier with the words "he needs bulletproof vests not an alternative voting system".

"We welcome the fact that two independent bodies have confirmed that our advert complies with the guidelines on political advertising," a No spokesman said.

"It is quite amazing that the Yes campaign have actually tried to prevent us revealing the £250m cost of the Alternative Vote to the public. If Yes to AV don't like the figure they should publish their own estimate of the cost and openly debate it."

The ASA said that "for reasons of freedom of speech", it did not have powers to adjudicate on print adverts "where the purpose of the ad is to persuade voters in a local, national or international election or referendum".

Separately, the No campaign have urged the authorities to investigate the role of the Electoral Reform Society (ERS) in the pro-change campaign following claims in the that the campaign group may be facing a conflict of interest over the referendum.

The ERS is a shareholder in Electoral Reform Services Ltd, which provides ballot papers, polling cards and registration forms for general elections and council polls among other services.

The No campaign said ERS - which Prime Minister David Cameron has claimed is "bankrolling" the Yes campaign - has potentially "stood to benefit financially from the outcome" of the poll.

But the organisation said it should be "no surprise" that it was supporting a Yes vote given its "longstanding commitment" to changing the voting system.

"The Society has openly declared the financial support we have provided to the Yes campaign - in sharp contrast to the No campaign who have so far refused to declare their financial backers," said its operations director Kate West.

"This is yet more noise designed to detract from the real issues at hand."