SACRAMENTO — California’s high-speed rail project is no longer subject to the state’s rigorous environmental laws after a federal transportation board ruled that it has oversight of the project, the state attorney general’s office argues in a brief filed Friday.

The June decision by the federal Surface Transportation Board — which was sought by opponents of the bullet train — pre-empts the authority of the California Environmental Quality Act, the state argued in the filing made on behalf of the California High-Speed Rail Authority.

“The STB’s decision concluding it has jurisdiction over the entire high-speed train system fundamentally affects the regulatory environment for the project going forward,” the state said in the brief submitted to the Third District Court of Appeals.

Opponents of the project could lose one of their most significant legal tools if a federal judge agrees with the state’s argument. Critics of the rail line have repeatedly sued the state alleging violations of Environmental Quality Act.

The state asked the court to dismiss a 5-year-old lawsuit filed by the San Francisco Bay Area cities of Atherton, Menlo Park and Palo Alto seeking to block the bullet train through the Pacheco Pass south of San Francisco. They argued that the route would harm the environment.

A Sacramento County Superior Court judge dismissed their suit in February but they appealed to the federal court, which last month ordered both sides to answer the question “Does federal law pre-empt state environmental law with respect to California’s high-speed rail system?”

The $68 billion project will have to comply with stringent environmental laws regardless of the court’s decision in the Atherton lawsuit. But if the court sides with the state, it would mean complying only with the National Environmental Policy Act, and any lawsuits would have to be filed in federal court.

An attorney for the cities, Stuart Flashman, was on vacation and unavailable, according to a voice mail recording at his office. He did not immediately respond to an email sent late Friday.

Rail authority Chief Executive Jeff Morales said there is “overwhelming overlap” between the two environmental laws, and that high-speed rail is committed to environmental protection even beyond the laws, such as requiring fuel efficient technology for construction and a carbon-neutral project.

“Those are things that aren’t strictly required under federal or state law and that would not change based on the outcome of this,” Morales said in an interview Friday.

Ironically, it was opponents of high-speed rail project, led by U.S. Rep. Jeff Denham, R-California, who first sought the federal oversight of the project, believing it would delay or even derail the bullet train by forcing it to comply with federal railroad regulations.

The Surface Transportation Board ruled in June that while it does have authority over the proposed 800-mile system, the rail authority could begin work on the first 65 miles of the project from Merced to Fresno if it maintains the current route and mitigates environmental damage caused by construction.

State lawmakers approved the first phase of the line last summer, allowing the state to begin selling $2.6 billion in voter-approved bonds for construction in the Central Valley. That approval allowed the state to tap $3.2 billion from the federal government, but it’s still unclear where the state will get the rest of the money.