A judge in San Francisco said she intends to strike a ban on male circumcision from the city's November ballot.

Superior Court Judge Loretta Giorgi said in a tentative ruling that the proposed law prohibiting circumcision of male children violates a California law that makes regulating medical procedures a function of the state, not cities.

"It serves no legitimate purpose to allow a measure whose invalidity can be determined as a matter of law to remain on the ballot," Ms Giorgi wrote.

Ms Giorgi ordered San Francisco's elections director to remove the controversial measure from the ballot that would have made the city the first in the nation to hold a public vote on whether to outlaw the circumcision of minors.

The citizens' initiative, which made the ballot in May after supporters gathered the required 7,163 signatures, would have made the practice a misdemeanour offence punishable by a fine of up to 1,000 US dollars or up to one year in jail.

The initiative did not offer exemptions for religious rituals such as the Jewish bris or Muslim khitan.

The city attorney's office, which had joined several Jewish organisations in challenging the ban in court, said Ms Giorgi plans to hear arguments on the issue before making her ruling final.

Backers had argued the ban was necessary to prevent a form of genital mutilation from being forced on children.

Critics contended the initiative posed a threat to constitutionally protected religious freedoms and cited comic books and trading cards distributed by the measure's proponents that carried images of a blonde, blue-eyed superhero and four evil Jewish characters.

The ban's sponsor, anti-circumcision activist Lloyd Schofield, said that he disagreed with the judge's interpretation and deliberately crafted a local ordinance "because for 10 years, no one on a state-wide or national level would even consider this".

Even if San Francisco voters will not be able to make their views known, the attention surrounding the ballot initiative has been good for circumcision opponents, Mr Schofield said.

"The mission won't be accomplished until men enjoy the same protection as women from forced genital mutilation, but everything we can achieve on the way to that goal is a step in the right direction," he said.

"Just getting people to think and discuss this is very rewarding because people don't think about this, they just do it automatically."

Though state law prohibits cities and counties from regulating medical procedures, two California Assembly members have introduced a bill that would specifically pre-empt local governments from enacting laws regarding male circumcision.