A vicious row has broken out between Yes and No campaigners over a lack of transparency in funding sources.

Yes campaigners have long suspected the influence of right-wing Christian groups from the US, such as the National Organisation for Marriage (NOM) who this week published a letter urging supporters to visit keepmarriage.org, a site advocating a No vote.

“Just like in campaigns for marriage here in America,” reads the letter, “slanted public opinion polls become fodder to influence and depress supporters of marriage. This is happening in Ireland. If [the no campaign] can manage to pull off a victory, it will be a tremendous boost to the cause of marriage worldwide. Please do what you can to bring awareness to their efforts.”

However, a NOM spokesman denied that it had channelled funds to any of the No groups, stating the group is aware that funding from foreign donors to lobby groups is prohibited.

Despite no discernible income source, the No campaign have planned full-page advertisements in national newspapers, additional billboards and a proliferation of posters in the final week leading up to the referendum.

Meanwhile, the Yes camp hit back at claims that they have received funding from US group Atlantic Philanthropies, a charitable organisation that has been a long-time supporter of Ireland’s LGBT community.

“Atlantic Philanthropies are not funding the Yes Equality referendum campaign. Yes Equality is fully funded through its supporters organising fundraising initiatives throughout Ireland,” GLEN spokesman and Yes Equality co-director Brian Sheehan told The Guardian.

“In addition we ran a crowd-funding campaign to raise monies for our poster, bus tour and booklet campaigns. All elements of the Yes Equality campaign are appropriately registered with the Standards in Public Office commission.”

“Yes Equality is entirely dependent on generous small donations from around the country. The average donation made to Yes Equality has been €70,” he added.