California will "imminently" challenge US President Donald Trump's declaration of a national emergency to obtain funds for a US-Mexico border wall, state Attorney General Xavier Becerra has said.
"Definitely and imminently," Mr Becerra told ABC's This Week program when asked whether and when California would sue the Trump administration in federal court.
Other states controlled by Democrats are expected to join the effort.
"We are prepared, we knew something like this might happen. And with our sister state partners, we are ready to go," he said.
Mr Trump invoked the emergency powers on Friday under a 1976 law after Congress rebuffed his request for $5.7 billion to help build the wall that was a signature 2016 campaign promise.
The move is intended to allow him to redirect money appropriated by Congress for other purposes toward wall construction instead.
The White House says Mr Trump will have access to about $8 billion.
Nearly $1.4 billion was allocated for border fencing under a spending measure approved by Congress last week, and Mr Trump's emergency declaration is aimed at giving him another $6.7 billion for the wall.
Mr Becerra cited the US president’s own comment on Friday that he "didn't need to do this" as evidence that the emergency declaration is legally vulnerable.
"It's become clear that this is not an emergency, not only because no one believes it is but because Donald Trump himself has said it's not," he said.
Mr Becerra and California Governor Gavin Newsom, both Democrats, have been expected to sue to block Mr Trump's move.
Mr Becerra told ABC that California and other states are waiting to learn which federal programs will lose money to determine what kind of harm the states could face from the declaration.
He said California may be harmed by less federal funding for emergency response services, the military and stopping drug trafficking.
"We're confident there are at least 8 billion ways that we can prove harm," Mr Becerra said.
Three Texas landowners and an environmental group filed the first lawsuit against Mr Trump's move on Friday, saying it violates the Constitution and would infringe on their property rights.
The legal challenges could at least slow down Mr Trump's efforts to build the wall but would likely end up at the conservative-leaning US Supreme Court.
Congress never defined a national emergency in the National Emergencies Act of 1976, which has been invoked dozens of times without a single successful legal challenge.
Democrats in Congress have vowed to challenge Mr Trump's declaration and several Republicans have said they are not certain whether they would support the president.
"I think many of us are concerned about this," Republican Senator Ron Johnson of Wisconsin, who chairs the Senate Homeland Security Committee, told NBC's "Meet the Press."
Mr Trump could, however, veto any resolution of disapproval from Congress.
White House senior adviser Stephen Miller told Fox News that Mr Trump's declaration would allow the administration to build "hundreds of miles" of border wall by September 2020.
"We have 120-odd miles that are already under construction or are already obligated plus the additional funds we have and then we’re going to outlay - we’re going to look at a few hundred miles."
Mr Trump's proposed wall and wider immigration policies are likely to be a major campaign issue ahead of the next presidential election in November 2020, where he will seek a second four-year term.