He denied Mr Cameron had distanced himself from the No campaign, saying the prime minister was "articulating the divide between No to AV and himself".

BBC: AV referendum: Cameron 'not responsible for No to AV'

Mr Johnson and Labour leader Ed Miliband are among Labour MPs backing a change to AV - but many heavyweight figures within the party, including former Deputy Prime Minister John Prescott, and most Conservatives, are campaigning to keep the current first-past-the-post system for Westminster elections.

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

Many Tories, who are opposed to AV, want to exploit the unpopularity of Nick Clegg, the Lib Dem deputy prime minister who champions voting reform.

ECONOMIST: The politics of the AV referendum