The Yes campaign has accused its rivals of "lies", saying the claims are largely based on the alleged cost of introducing electronic counting machines and that these are not required for AV elections.

BBC: Q&A: Alternative vote referendum

The letter was organised by the historian turned Conservative MP Chris Skidmore, not the official No to AV campaign.

BBC: AV referendum: Union stance 'depressing', says Johnson

For his part, Mr Cameron opposes AV, but might not campaign against it as vigorously as many of his Tory colleagues will.

ECONOMIST: Voting reform and the coalition