View of a section of the U.S.-Mexico border fence at El Paso, U.S. Thomson Reuters SAN FRANCISCO (Reuters) - California's attorney general plans to file a lawsuit on Wednesday challenging President Donald Trump's plan to construct a wall along the border with Mexico, the state AG's office said, adding to the obstacles facing a key Trump campaign promise.

Trump has insisted Mexico would pay for building the wall, which experts said could cost about $22 billion and take more than three years to complete.

With Mexico refusing to pay, Trump has said since taking office in January that the wall will initially need US funding but that he will find a way to make Mexico ultimately pay for it.

Democrats in the US Congress, however, firmly oppose the border wall, and at least some Democratic senators would need to vote for its inclusion in a spending package.

Democratic attorneys general including California's Xavier Becerra have sued the Trump administration on a range of issues.

The border wall lawsuit set to be filed on Wednesday will allege that Trump's wall violates federal environmental standards, as well as constitutional provisions regarding the separation of powers and states' rights, a Becerra spokesperson said.

Last month the Trump administration said it had selected four construction companies to build concrete prototypes for a wall, which will be will be 30 feet (9 meters) tall and about 30 feet wide and will be tested in San Diego.

(Reporting by Dan Levine; Editing by Sandra Maler)