Ballot papers for the Tory leadership contest are going out to more than 300,000 party members across the country.
It is the largest exercise in popular democracy the Conservatives have ever staged.
Mr Iain Duncan Smith is pollsters' and bookies' favourite to defeat Mr Kenneth Clarke on September 12th.
But with even party bigwigs unsure of exactly how many members are entitled to take part in the postal ballot, predictions of the result are thought to be more than usually unreliable.
Both candidates are set to publish their manifestos for the leadership battle.
Mr Clarke's is said by his team to focus on health, education, transport and the police.
Mr Duncan Smith's manifesto commits him to a tax-cutting agenda and promises a get-tough programme of zero-tolerance policing of petty crime and stronger discipline in school.
It pledges his opposition to the euro and the so-called Euro-army and support for US President Mr George W Bush's missile defence project.
PA
