A second religious controversy in as many days is distracting the electoral reform campaign.
The Yes campaign was forced to distance itself from a campaigner who had tweeted a pro-alternative vote (AV) joke on microblogging site Twitter.
Ben Donnelly tweeted yesterday afternoon: "Says in the Holy Qu'ran Mohammad used to get his neighbours to vote by AV which of his 4 wives he'd shag each night."
Labour's Khalid Mahmood told the Evening Standard newspaper the comments were "outrageous and totally Islamophobic". A Yes Campaign spokesperson said the comments were "utterly disgraceful".
It followed scornful reactions yesterday after Church of England figures came out in support of AV.
The Bishop of Exeter, the Rt Rev Michael Langrish, was quoted as saying that "voting systems are not values-free" as he endorsed a switch away from first-past-the-post "on the grounds of justice and accountability".
The bishops of St Albans, Leeds, Ipswich, Buckingham, Gloucester all joined him in offering support for AV. Four former bishops were also named as AV backers.
"The bishops are frustrated that there is very little in the way of the moral, ethical dimension to this," Yes to Fairer Votes spokesman Jonathan Bartley said.
"The debate has been constructed so far in terms of which party will benefit and who is trying to gain an advantage out of this. What the bishops are saying is that there are more important issues."
But Jane Kennedy, a spokeswoman for the No campaign, said she was concerned at the implication that it might be "immoral" to oppose the "deeply-flawed" AV system of voting.
"This referendum requires a sober, sensible analysis of the case for and against scrapping our current electoral system," she said.
"There are people of all faiths and none who are deeply unhappy about the proposal. The Yes campaign should not attempt to turn this debate into a moral crusade."
The Rt Rev Colin Buchanan, former bishop of Woolwich, is a vice-chair of the Electoral Reform Society which is donating up to £1.5 million to the Yes campaign, the No campaign said.
The developments followed a compromise on the parliamentary voting system and constituencies bill earlier this week, ending speculation that the referendum might not go ahead.
The bill must be passed by February 26th if the vote - scheduled for May 5th - is to go ahead, but after ministers made concessions to Labour peers' demands it seems all but certain to pass parliament's upper House.