President Trump has confirmed he will use emergency powers to build a wall on the US border with Mexico, saying “walls work”.
Building the wall was a key pledge of Mr Trump’s campaign, but Democrats have described the emergency as a “gross abuse of power”.
He is due to sign the plan along with a spending bill aimed at preventing a repeat of a recent government shutdown.
He announced the plan after Congress refused to pay for a wall in the bill.
However, senior Democrats said immediately that they would challenge the move in the courts.
The declaration will give Mr Trump access to billions of dollars for his project.
What did Mr Trump say?
Making the announcement to journalists in the White House Rose Garden, the president said the emergency would allow him to get almost $8bn for the wall.
“We’re going to confront the national security crisis on our southern border,” Mr Trump said.
“We have an invasion of drugs, invasion of gangs, invasion of people, and it’s unacceptable.
“Everyone knows that walls work.”
But Mr Trump accepted that he would be sued for the move, and predicted that the emergency order would lead to legal action which was likely to end up in the Supreme Court.
By Jon Sopel, BBC North America editor
The trouble with going nuclear is there is fall-out. This has been presented as a predictably partisan issue.
On one side of the wall, Republicans; on the other side Democrats. But by going nuclear the president has made it more complicated than that. There are a lot of Republicans – in the Senate and in the House – deeply uneasy about what Mr Trump is doing.
Why? Because the constitutional arrangement of the US is that Congress controls the purse strings and allocates funds.
Not the president. This is a major land grab by the president. It undermines their position and sets a very dangerous precedent.
How have Democrats responded?
House of Representatives Speaker Nancy Pelosi and Senate Democrat leader Chuck Schumer responded to Mr Trump’s announcement with a statement saying they would challenge the move in Congress and in the courts “using every remedy available”.
“The president’s unlawful declaration over a crisis that does not exist does great violence to our Constitution and makes America less safe, stealing from urgently needed defence funds for the security of our military and our nation,” the statement said.
“This is plainly a power grab by a disappointed President, who has gone outside the bounds of the law to try to get what he failed to achieve in the constitutional legislative process.”
Also, within minutes of the announcement, New York State attorney-general Letitia James vowed to mount a legal challenge.