WASHINGTON - US President Donald Trump has the power to abolish two national monuments in California established by his Democratic predecessor Joe Biden, as well as any others created by past presidents, according to a newly released legal opinion from a key adviser within the US Department of Justice.

The May 27 document, which was released on June 10, reversed a 1938 legal opinion and sets the stage for the Republican president to eliminate federal protections for potentially millions of acres of land previously designated as national monuments.

National monuments are created by presidents in recognition of a site's cultural, historical or scientific importance, while national parks are created by Congress largely to protect outstanding scenic features or natural phenomena.

Ms Lanora Pettit, who heads the Justice Department's Office of Legal Counsel, was asked by the White House to provide a new opinion as Mr Trump considered whether to reverse Mr Biden's decision in January in his final days in office to designate two California sites that hold significance to Native American tribes as national monuments.

The Chuckwalla National Monument preserves more than 624,000 acres just south of Joshua Tree National Park.

The Sattitla Highlands National Monument protects 224,000 acres where the dormant Medicine Lake volcano carved craters and lava tubes.

Mr Biden had relied on the Antiquities Act of 1906, a law that has been invoked by numerous presidents to designate over 100 national monuments.

A 1938 opinion by Attorney-General Homer Cummings, who served under Democratic President Franklin Roosevelt, has been long cited as constraining the ability of presidents to undo past designations.

But in a 50-page legal opinion, Ms Pettit concluded that the Antiquities Act gives presidents not only the power to establish national monuments on federal lands but also to determine that they never were or no longer are deserving of those protections.

The White House has also asked Ms Pettit to consider whether the Justice Department should disavow Cummings' prior opinion.

She said it should, writing that the law's silence about whether a president could revoke a predecessor's designation of a monument should be understood as meaning he has such authority.

"Thus, for the Antiquities Act, the power to declare carries with it the power to revoke," Ms Pettit wrote.

The legal opinion did note that since the earliest days of the Antiquities Act, presidents have “from time to time diminished”...