Up to 1,000 Tory party members in Bromley and Chislehurst rejected candidates on Mr Cameron's A-list and chose Bob Neill, leader of the Conservatives on the London Assembly, to represent them.
The by-election has been prompted by the death of the sitting MP, Eric Forth, who died suddenly from cancer last month.
The seat is seen as a key Tory stronghold, where the majority at the 2005 general election was 13,342. Mr Forth's widow, Carroll, entered the race to succeed him, but did not make it to the final shortlist.
Mr Neill, 53, who represents the area on the London Assembly, saw off A-list candidates Julia Manning, chair of the Conservative Christian Fellowship, and Syed Kamall, an MEP, in the final run-off.
Last night, the Conservative Party put a brave face on the choice and denied that it was a snub. A spokesman said A-listers need not be chosen to fight by-elections, but would be selected to fight the next general election.
Bernard Jenkin, the Tory vice-chairman, said: "We are absolutely delighted that Bob has been selected.
"He is a natural successor to Eric Forth with his unparalleled experience and local knowledge."
Mr Neill, who is expected to face a challenge from the Liberal Democrats for the seat, said: "It is a great honour to be selected to fight Bromley and Chislehurst. I have enjoyed representing the area for six years on the London Assembly and I hope to have the opportunity to do so as its Member of Parliament."Reuse content