Newport Beach officials failed to adequately review a large proposed development on the coastal Banning Ranch oil field before approving it, the California Supreme Court ruled Thursday.

The unanimous decision, written by Justice Carol A. Corrigan, was another blow for the developers whose project was rejected last year by the California Coastal Commission.

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Deborah Sivas, a Stanford University professor of environmental law, said the decision affirmed standing legal doctrine. “This court is not stepping out,” she said. “It’s in the mainstream.”

Sivas added that the Banning Ranch decision signaled that the Supreme Court “is going to be pretty sympathetic in interpreting” requirements under the California Environmental Quality Act, the state’s key environmental law.