The US government is introducing a new fast-track deportation process that will bypass immigration courts.
Under the new rules, any undocumented migrants who can't prove they have been in the US continuously for more than two years can be immediately deported.
The policy is expected to be published on Tuesday, and then implemented across the country with immediate effect.
However, the American Civil Liberties Union (ACLU) rights group has said it plans to challenge the policy in court.
It comes as US immigration policy comes under increasing scrutiny - in particular, the conditions at the country's detention centres on the southern border with Mexico.
Kevin McAleenan, acting secretary of Homeland Security, said the change would "help to alleviate some of the burden and capacity issues" at the border.
He added that it was "a necessary response to the ongoing immigration crisis".
Analysts say US President Donald Trump is planning to make hardline immigration control a key element of his re-election campaign in 2020.
What's changing?
Previously, only people detained within 100 miles (160km) of the border who had been in the US for less two weeks could be deported quickly.
Migrants who were found elsewhere, or who had been in the country for more than two weeks, would need to be processed through the courts and would be entitled to legal representation.
But the new rules state that people can be deported regardless of where in the country they are when they are detained, and without allowing them access to a lawyer.
The Department of Homeland Security said the new rules would allow it to pursue large numbers of illegal migrants more efficiently.
Migrants who are eligible for asylum will still be entitled to speak to an asylum officer, it added.
What has the response been? Within hours of the policy being announced on Monday, ACLU said that it was planning to launch a legal challenge.
"We are suing to quickly stop Trump's efforts to massively expand the expedited removal of immigrants," the rights group tweeted.
"Immigrants that have lived here for years will have less due process rights than people get in traffic court. The plan is unlawful. Period."
Vanita Gupta, president of the Leadership Conference on Civil and Human Rights, told reporters: "The Trump administration is moving forward into converting ICE [Immigration and Customs Enforcement] into a 'show me your papers' army."
Legal expert Jackie Stevens, a political science professor at Northwestern University, told Reuters that about 1% of the people detained by ICE and 0.5% of those deported were actually US citizens.
"Expedited removal orders are going to make this much worse," she said.
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