In an unprecedented move that will likely face a hefty legal challenge, the president will order that government money be directed towards his promised barrier.
Trump will also sign off on a spending bill agreed by lawmakers earlier this week that will avoid a second crippling government shutdown.
White House spokeswoman Sarah Sanders said: "President Trump will sign the government funding bill, and as he has stated before, he will also take other executive action – including a national emergency – to ensure we stop the national security and humanitarian crisis at the border."
The emergency declaration will infringe upon the power of Congress over control of the national purse strings.
And it will plunge Trump into an extended court battle with lawmakers over the limit of his power.
The top Democrat in Congress immediately denounced the president's move.
LEGAL FIGHT
Asked by reporters if she would file a legal challenge to an emergency declaration, House of Representatives Speaker Nancy Pelosi said: "I may, that's an option."
And the top Senate Democrat, Chuck Schumer, accused Trump of a "gross abuse of the power of the presidency."
Senate Majority Leader Mitch McConnell, a Republican, said he will support Trump on an emergency declaration.
Trump had asked Congress to approve $5.7billion to fund around 250 miles of border wall construction.
His administration has presented the barrier as vital to prevent a "crisis" at the frontier with Mexico.
But Democrats stood in the way of approving the taxpayer money – viewing the wall as an expensive and immoral waste.
Moving to an emergency declaration is a next step that Trump has been exploring for weeks.
The White House has reportedly identified $2.7bn in funds previously provided by Congress that could be redirected to barrier funding as part of a national emergency, sources said.
White House lawyers had vetted the figures and believed they would withstand a legal challenge, the source added.
SHUTDOWN AVERTED
The government spending bill has passed the Senate and is expected to be approved by lawmakers in the House of Representatives on Thursday.
It would provide more than $300bn to fund the Department of Homeland Security and a range of other agencies through to 30 September, the end of the current fiscal year.
The legislation includes $1.37bn in new money to help build 55 miles of new physical border barriers.
That is the same level of funding Congress appropriated for border security measures last year, including barriers but not concrete walls.
Funding for those agencies is due to expire on Friday, which would trigger another partial federal shutdown on Saturday morning if Congress and Trump do not act quickly.
'END-RUN'
Pelosi accused Trump of doing "an end-run" around Congress and around the Constitution's separation of powers that gives Congress, not the president, federal spending authority.
She said: "It's not an emergency, what's happening at the border. It's a humanitarian challenge to us."
Some of Trump's fellow Republicans have warned him that declaring a national emergency could set a dangerous precedent.
It opens the door for a future Democratic president to circumvent Congress and declare emergencies on issues like climate change or healthcare insurance.
Pelosi said: "If the president can declare an emergency on something that he has created as an emergency – an illusion that he wants to convey – just think of what a president with different values can present to the American people."
She cited gun violence in the United States as an emergency.
Democratic Senator Ron Wyden said: "I will pull out all the stops to stop this horrendous, bizarre idea.
"As people get a chance to sort through what the implications really are, we will have strong bipartisan support for opposing this.
"We are the appropriators, not the president."
Trump triggered a 35-day-long shutdown of about a quarter of the federal government with his December demand for $5.7bn to help build a portion of the wall.
He was widely blamed for the shutdown and agreed to end it without getting the wall money.
The legislation being acted upon on Thursday also funds the Justice Department, Commerce Department, State Department, Department of Agriculture, Internal Revenue Service and others, covering roughly 800,000 federal workers.
National emergency: How Trump could secure border wall funding with presidential declaration
As early as last March, Trump was publicly floating the idea of using the military for the task.
"Building a great Border Wall, with drugs (poison) and enemy combatants pouring into our Country, is all about National Defense. Build WALL through M!" he tweeted.
But it's Congress not the president that controls the country's purse strings and must appropriate money he wants to spend.
Among the laws Trump could turn to is Section 2808 of Title 10 of the U.S. Code pertaining to military construction.
According to the statute, if the president declares an emergency "that requires use of the armed forces," the Defense secretary "may undertake military construction projects, and may authorize the Secretaries of the military departments to undertake military construction projects, not otherwise authorized by law that are necessary to support such use of the armed forces."
Another law, Section 2293 of the code's Title33, allows the diversion of funds from an Army civil works project to a mission that is "essential to the national defense."
Congressional aides say there is $21 billion in military construction funds that could potentially be used for a wall in the event Trump declares an emergency.
There is about $10 billion in funds from the current 2019 fiscal year that ends on 30 September, and another $11 billion from the previous four years that haven't been obligated or contracted for a project, the aides said.
They spoke on condition of anonymity because they weren't authorized to speak publicly about the funding details.
The Defense Department has declined to provide any details on the amount of money available.
The congressional aides said their data came from the Pentagon.
As an example, the aides said there is funding for a medical facility at a U.S. base in Germany that has been partially built.
If those funds are used, the medical center could be left half built.
The aides said that while the president can decide to use military construction funds, it would likely be up to the Defense Department to determine which specific projects would lose their money.
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