The Oklahoma Supreme Court unanimously vetoed a ballot measure that would have given embryos full personhood rights on Monday, ruling it "clearly unconstitutional" because it would block a woman's legal right to have an abortion.

The personhood measure would give embryos the same legal rights as people from the moment of fertilization, which opponents say would ban abortion and complicate the legality of in vitro fertilization and many forms of birth control. Enacting such a law would violate the U.S. Supreme Court's 1992 decision in Planned Parenthood v. Casey, which allows states to pass some abortion restrictions but prevents them from overturning the constitutional right to end a pregnancy.

"The mandate of Casey is as binding on this Court today as it was 20 years ago," the Oklahoma Supreme Court wrote in its decision. "Initiative Petition No. 395 conflicts with Casey and is void on its face and it is hereby ordered stricken."

A personhood bill also failed in the Oklahoma legislature earlier this month, when Republican leadership in the state House of Representatives decided not to bring it to a vote, despite mounting pressure from anti-abortion groups.

“This amendment would have run roughshod over the fundamental, constitutionally protected reproductive rights of all Oklahoma women," said Nancy Northup, president and CEO of the Center for Reproductive Rights, which filed the case against the measure. "In affirming our request to strike it down, the court has struck a powerful blow against the repugnant tactics of those who seek to vote down the rights of others, and to enshrine their hostility to women’s lives, health, and rights in the laws of the land."