A federal judge in California has issued a temporary injunction blocking the use of Defense Department funds to build parts of President Donald Trump’s U.S.-Mexico border wall.

Oakland-based Judge Haywood Gilliam Jr. of the Northern District of California issued the order Friday in response to a lawsuit filed by the American Civil Liberties Union on behalf of the Sierra Club and the Southern Border Communities Coalition. Construction of sections of the wall affected by the ruling could have begun as early as Saturday.

The decision blocks the $1 billion transfer of Pentagon “counter-drug” funds so they can’t be used for wall construction. Federal officials are prevented from “taking any action to construct a border barrier in the areas.”

But it only applies to two projects to add 51 miles of fence in on the Mexican border in El Paso and in Yuma, Arizona. It also does not prevent the Trump administration from seeking other funding sources for those projects.

An additional $1.5 billion in funds from the same Pentagon program being considered for wall construction may also be impacted by the ruling.

Congress appropriated only $1.375 billion for 55 miles of a barrier limited to a section in the Rio Grande Valley in Texas. Trump then declared an emergency in a bid to obtain federal funds —including money from the Defense Department — without congressional approval.

“The position that when Congress declines the Executive’s request to appropriate funds, the Executive nonetheless may simply find a way to spend those funds ‘without Congress’ does not square with fundamental separation of powers principles dating back to the earliest days of our Republic,” the judge wrote.

Gilliam cited Congress’ “absolute” control over federal expenditures under the Constitution, “even when that control may frustrate the desires of the executive branch regarding initiatives it views as important.”

The ruling quoted from a Fox News interview with Mick Mulvaney, acting White House chief of staff, in which he declared that the wall “is going to get built, with or without Congress.”

The lawsuit had argued that Trump’s emergency declaration to obtain Pentagon funds for the wall was “made solely out of disagreement with Congress’ decision about the proper funding level, location, and timetable for constructing a border wall.” It also argued that the wall would negatively impact the environment and communities along the border.

Gilliam noted that the administration’s delay in using funds from the previous year undercut Trump’s claims that building a wall was an emergency.