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

Asked if it was possible the Lib Dems would make a deal to accept AV, which is not proportional, Mr Hughes said: "I don't think it's conceivable".

BBC: Hughes issues Lib-Lab ultimatum

When legitimate web sites are compromised with scripts launching fake AV dialogues, these rules do not apply.

FORBES: The Proliferation Of Scareware Hits Home

Eager to prop up his troubled deputy, he will not want to be seen as instrumental in defeating AV, should it be rejected.

ECONOMIST: Voting reform and the coalition

The experience in Australia, which has used AV since 1918, suggests that it would not be a disaster.

ECONOMIST: Constitutional reform in Britain

AV, but he points out that Mr Blair has already admitted that it is not a proportional system.

ECONOMIST: What's Cooking?