The Local Agency Formation Commission, a regional board overseeing secession, voted 8-1 on Wednesday to allow the measure to be placed on the Nov. 5 ballot.
A majority of residents of the proposed valley city and the remainder of Los Angeles must vote in favor of secession to create the new city. Major James Hahn suggested the city would file suit to block the vote.
"There is no rush here. You have plenty of time to do this right," Hahn said in a radio interview.
A San Fernando Valley city would have about 1.3 million residents and compete with Phoenix to be the sixth largest in the country. It would be larger than Boston, Detroit and St. Louis. The shrunken Los Angeles would become the nation's third-largest city, behind New York and Chicago.
"The only secession anywhere near this size that's ever been done in this country, I believe was called the Civil War," said Larry Calemine, executive director of the board.
Secessionist advocates complain that the 270-square-mile valley contributes more in city taxes than it gets back in services and is ignored by downtown politicians.
Opponents, including Hahn, argue that secession would be financially devastating to both Los Angeles and the new valley city, resulting in higher taxes and reduced services.
The panel made its decision after a seven-hour meeting that featured testimony from citizens, lawyers and elected officials.
Secession movements have flared repeatedly since Los Angeles annexed the valley in 1915. State laws quashed two serious secession movements in the 1960s and '70s.
Secession movements also are under way in Hollywood and the San Pedro community at the city's harbor. The commission is expected to decide whether to put secession to a vote in those areas in early June.
By Robert Jablon