That gives Labour an opening: though the party actually backed AVin its 2010 election manifesto, it now opposes any bill that involves what it calls gerrymandering.
However 11 leading businessmen have endorsed AVin a rival letter to the Daily Telegraph - in a move which was independent of the official Yes campaign.
Mr Johnson told BBC One's This Week programme the low point of his week had been "the rather depressing news that every trade union, apparently, is going to oppose AVin the referendum".