Prime Minister David Cameron has told one of the BBC's most senior journalists to go "back to school" after accusing him of failing to understand the electoral reforms being voted on in Thursday's referendum.
Mr Cameron said it was "worrying" that the Radio 4 Today programme's lead interviewer, John Humphrys, had given listeners incorrect information about how the alternative vote (AV) system works.
Mr Cameron's outburst came as he was interviewed about the May 5 vote on ditching first past the post for the AV system, which involves voters ranking candidates for Westminster elections in order of preference.
Under AV, the candidate receiving the fewest votes in each round is eliminated and the second preferences of his or her supporters redistributed between those remaining until one of them has more than 50% backing.
If the count goes to several rounds, this means backers of fringe parties could have their vote counted for a number of different candidates, while those supporting the eventual winner will have it counted each time for the same person.
Mr Cameron said this would result in a system where "you start counting some people's votes more than once", but Mr Humphrys retorted: "No, you don't. That simply isn't true, that you count some votes more than once."
The Prime Minister replied: "Yes, you do. You count all the votes, and then you start eliminating candidates and then you count people's second preferences."
Mr Humphrys said: "And I have a second preference as well as you or anybody else and you count them again as well, so you don't count some people's votes more than others."
Mr Cameron said: "You are wrong. If you vote for the Labour candidate and I vote for the Monster Raving Loony candidate and the Monster Raving Loony comes last, my second preference is then counted again," which Mr Humphrys replied: "So is mine".
Mr Cameron said: "No, it isn't. That's where you are wrong. It is quite worrying if actually the lead broadcaster on the BBC doesn't understand the system. You don't understand the system you are supposed to be explaining to the public. I do think that's worrying. Back to school."