The two camps on either side of the referendum on the alternative vote are trading blows over whether the group funding the yes campaign stands to gain financially from a switch to AV.

Campaigners for a no vote obtained a leaked document which they said proved one of the yes campaign's major backers was reliant on a company that could profit financially from a yes vote on 5 May.

Allegations by the no campaign that their opponents have a conflict of interest have dogged the debate all week, galvanised by an intervention by George Osborne on Monday when he suggested the yes campaign's funding model was "dodgy". He was reprimanded by lawyers acting for the yes campaign who said this was "wholly untrue".

Ed Howker, a journalist, obtained what appears to be an internal risk assessment by the Electoral Reform Society – the principal donor to the Yes to AV campaign – in which the ERS acknowledges that the targeting of its funding relationship was a "medium" likelihood with a "high impact".

In a section detailing the "issue" the body notes that it is reliant on cash advances from its sister company Electoral Reform Services Limited (ERSL) about which it writes: "It is possible that ERSL will profit as a result of a YES vote (increased business opportunities)."

Under a heading marked "risk" it writes: "Damage to reputation; could make us look unsavoury even if unfounded. Negative effect on donations levels if story becomes 'we are rich'? ERSL fear that negative publicity might affect union clients, Conservative party or other 'no' supporters which in turn, might affect future dividends."

The no campaign says the ERS has given £1.1m to the pro-AV campaign and claimed that the society and its subsidiaries had received more than £15m in contracts from the public purse over the past three years. The no campaign also charges that the ERSL would provide new telling machines for processing AV ballots if there is a switch to the new system.

The lawyers Lewis Silkin said on Monday: "This is wholly untrue. Electoral Reform Services Limited (the business arm of ERS) earns revenue in the public election administration area from three types of contract. Printing of ballot papers and the producing of voting packs for postal voters; printing and mailing of the annual canvas return forms and processing telephone and internet responses.

"Provision of election management software through its subsidiary Xpress software solutions. The form of voting system upon which parliamentary elections are based is entirely irrelevant to the provision of any of these services. A change in the voting system would, therefore, have absolutely no impact on any of the revenue earned by the ERSL."

George Eustice, the Tory MP co-ordinating the no campaign, said: "Yesterday, lawyers for the Electoral Reform Society issued a threatening letter in a bid to discourage the media from reporting their conflict of interest in this referendum. Now it turns out their own internal documents highlight that just such a conflict of interest exists and that they have wilfully misled British voters and the media.

"This appears to be a disgraceful attempt to influence the result of the referendum just as postal votes land on people's doormats. This is not how we do politics in this country."

Responding to the publication of the leaked document, the Electoral Reform Society said: "This was a scenario planning document anticipating lines of attack we've come under from the no campaign. ERSL is an independent company and have made perfectly clear they won't make a penny from a yes vote."