Campaigners against electoral reform today pledged to reveal details of their main financial backers before the country goes to the polls in May.
The No2Av campaign had come under pressure to reveal details of its funding sources – thought to be a mixture of union money and big Conservative donors - earlier this week.
The campaign today said the delay in doing so was due to the need to get permission from all donors to release the details which will eventually be published by the Electoral Commission – but not until after the election.
The development came as David Cameron and Nick Clegg waded into the AV campaign for the first time.
The Prime described the idea that AV was fair and proportional as "a myth", insisting it would mean some people's votes counted more than others'.
"If you vote for a mainstream candidate who is top of the ballot in the first round, your other preferences will never be counted," he said.
"But if you vote for a fringe party who gets knocked out, your other preferences will be counted. In other words, you get another bite of the cherry.
"I don't see why voters of the BNP or Monster Raving Loony Party should get their votes counted more times than supporters of the Conservatives or, for that matter, Labour or Liberal Democrats."
But Mr Clegg insisted that existing voting system was responsible for encouraging politicians to abuse expenses.
"It is because there are so many MPs with jobs for life that there are so many who can take their constituents for granted,” he said.
"And it is because there were so many MPs taking their constituents for granted that so many abused their expenses.
"When a person is corrupt, they should be punished. When a system makes corruption more likely, it should be changed."
He also said AV would make MPs "work harder”.
"It means that parties will have to compete for votes in every corner of the country and not just those few marginal seats," he said.
"It means more people get listened to and more respect for the different opinions and feelings we share as a nation."
Mr Clegg dismissed objections that AV was too complicated, pointing out that the system of ranking candidates by preference was already used in Australia and London mayoral elections.
Both men stressed that the result of the ballot on May 5 would have no effect on the stability of the coalition.
But answering questions after his speech, Mr Cameron said he would have preferred to hold the referendum later in the Parliament.
However, he decided to accept the date because there were "things I needed to achieve through the coalition".
"It's absolutely no secret that I did not want the referendum to take place on May 5 - I was quite keen for it to happen later in the Parliament," Mr Cameron said.
"This was extremely important to the Liberal Democrats.
"There are things I need to achieve through the coalition and in the end I decided it was right to have this referendum on May 5.
"If there is a No vote, the Liberal Democrats are able to say to their supporters 'We had the referendum we got in the coalition agreement and we had it on the day of our choosing'."