Early results from America’s most populous state indicate that a ballot measure to ban gay marriage may pass.

Proposition 8 aims to overturn the decision of the California Supreme Court that same-sex couples have the right to marry.

Results from 13% of polling stations show it leading by 54%.

Since the landmark court ruling in May an estimated 18,000 gay and lesbian couples have tied the knot, among them Hollywood stars Ellen Degeneres and George Takei.

A similar measure banning gay marriage passed in Florida this morning and gay activists are nervous as the polls suggest a dead heat.

The result will not be known for some hours, but early tallys suggest Prop 8 may pass.

“We can not face tomorrow morning with same-sex couples having had our right to marry eliminated because voters decided it was more important to get home and start dinner than to go vote,” Kate Kendell, executive director of the National Centre for Lesbian Rights, told ABC.