Mr Cameron said the excercise was an "exciting opportunity"
Every voter in Totnes will be able to help choose their Conservative general election candidate in an "open primary" contest, the Tories have announced.
Some 69,000 people will be invited to take part in a postal ballot to pick Anthony Steen's successor.
Tory leader David Cameron said: "This is the first time any political party in Britain has sought the views of the voters in such a direct way."
Mr Steen has said he would stand down following reports about his expenses.
The Daily Telegraph reported that he claimed more than £87,000 over four years for his country home.
Local party members in Totnes have already drawn up a shortlist of 11 would-be candidates to contest the next election.
This will be further reduced to a list of two to four hopefuls, who will then seek the support of the entire constituency's electorate.
All voters in the constituency will receive a ballot paper and a freepost envelope to return it in.
Mr Cameron said the exercise was an "exciting opportunity" to engage with the electorate.
He added: "I hope people will take part in this unique chance to have a real say over how politics works and who represents them."
The Conservative leader has previously spoken in favour of open primaries - based on the system used in some US states for selecting party candidates.
Other supporters of the process include Tory backbencher Douglas Carswell and Labour former minister Frank Field.
Foreign Secretary David Miliband has also spoken favourably about the system.
In 2007, London Mayor Boris Johnson was chosen as Conservative candidate in a contest which was open to all Londoners on the electoral roll who registered on a telephone hotline.