John Moodey has resigned as leader of the Democratic Alliance (DA) in Gauteng and as member of the party.
Moodey has been a member of the party since 1998.
He joins a string of prominent members who have left the DA since last October.
Former city of Johannesburg mayor Herman Mashaba and former party leader, Mmusi Maimane led the exodus from the party.
Moodey says he cannot be part of an organization which is comfortable with being the main opposition in the country.
In February this year, Moodey said he is the workable solution to lead the official opposition party. He was launching his campaign as the party leader at its elective conference in May: