Joe Biden is set to close the border with Mexico under a new executive order clamping down on asylum seekers.
Signed on Tuesday, the order allows the US to suspend entry of non-citizens crossing from Mexico once the average number of daily encounters hits 2,500 between official ports of entry.
A senior administration official said that under the order, the border would be shut down immediately as the threshold has already been met.
They added the border - which the White House says is "overwhelmed" - would only reopen once the number of migrant encounters at the US border falls to 1,500 a day.
Once the order is in effect, migrants who don't express fear at returning to their home countries will be subject to immediate removal from the US. They could also face a five-year ban on re-entering the country, and possibly be criminally prosecuted.
Speaking at the White House after signing the order, Biden said Republicans "have left me no choice" but to "do what I can on my own to address the border".
In a statement, the White House added the order has "humanitarian exceptions" including for unaccompanied children and victims of trafficking.
But the Biden administration does expect legal challenges against the order, with the American Civil Liberties Union (ACLU) already saying it intends to do so.
ACLU attorney Lee Gelernt said: "A ban on asylum is illegal just as it was when [Donald] Trump unsuccessfully tried it."
Lindsay Toczylowski, the executive director for the California-based Immigrant Defenders Law Center, also told Sky's partner network NBC: "It is a betrayal of what we were told in [Biden's] campaign four years ago.
"We were told that President Biden would be restoring humanity at our border... But what we are seeing is that history is repeating itself."