Lawyers for the city of San Francisco have suggested the proposed ban of circumcision for underage males contravenes the US constitution.

Their comments come after a challenge to the ban from an interfaith group of Muslims and Jews on the basis that Californian state law prohibits individual city lawmakers from placing limits on procedures by medical staff.

But the result of that would mean that a circumcision ban could only be enforced for non-medical staff – such as mohels – and would therefore potentially violate the First Amendment's guarantee of religious freedom.

The San Francisco Attorney's Office said: "San Franciscans cannot be asked to vote on whether to prohibit religious minorities from engaging in a particular religious practice, when the same practice may be performed under nonreligious auspices.

"If the court concludes that the measure is pre-empted as applied to medical professionals, then the remaining application is unconstitutional and the court should remove the measure from the ballot entirely."

As it stands, residents will be asked to vote on the ban at a ballot in November.