Ads by the No to AV campaign have courted controversy, suggesting that implementing the alternative voting system would suck £250m from the NHS and the armed forces.
Until now, most polls have shown the Yes and No camps at similar levels. But BritainThinks polled more than 2,000 people and found 46 per cent opposed to AV. Just 33 per cent were in favour.
The in-depth research also found that both camps are struggling to win over undecided voters.