Liberal Democrat Cabinet minister Chris Huhne has warned that dirty tactics in the Alternative Vote battle are damaging the coalition.
Mr Huhne accused the No campaign of "gutter politics" and "downright lies" in their claims about the costs of changing the electoral system.
"I am frankly shocked that coalition partners can stoop to a level of campaign that we have not seen in this country before," the Energy Secretary told the BBC's Newsnight programme. "I think it is damaging. There is no doubt about it. I can never remember a campaign that has stooped as low as the No campaign in dredging up stuff that they know is downright lies."
He added: "I think this is the politics of the gutter."
Mr Huhne also complained that he had still not received a reply to a letter he sent to Tory chairman Baroness Warsi earlier this month demanding an end to "scare and smears".
Both David Cameron and Lib Dem leader Nick Clegg have insisted that the coalition will survive regardless of the result of the May 5 referendum. But even before Mr Huhne's intervention, a day of unlikely cross-party alliances had demonstrated the potential for tensions.
Mr Cameron shared a platform with Labour former home secretary Lord Reid to call for AV to be rejected. Meanwhile, Labour leader Ed Miliband and Mr Cameron's Lib Dem Business Secretary Vince Cable joined forces to urge people to back the switch.
Although there is still little evidence of the campaign firing the public imagination, a poll suggests that those opposed to the change were faring better and had opened up a 16-point lead. ICM research for the Guardian found that among those saying they had made up their mind and were likely to cast a ballot, 58% were planning to vote No compared to 42% Yes.
Three-quarters of Conservatives are planning to vote against, as are a small majority of Labour supporters. Only Lib Dem voters are firmly in favour, with more than two-thirds saying they will vote for the change.
However, the Yes camp can take some solace in the 23% who say they still do not know how they will vote.