The Prime Minister and his deputy are to set out opposing views on the AV referendum.
Deputy Prime Minister Nick Clegg told Today presenter Sarah Montague that AV "is a small change that will make a big difference".
A change in the system was "a natural evolution", that would "make politicians work harder for your vote", he said.
But Foreign Secretary William Hague said AV was "the worst of all worlds".
"It is likely to produce election results which are more indecisive, or more disproportionate, or even both at the same time," he said.
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