The Trump administration will this week introduce a speedy deportation process that will bypass immigration courts.
Under the new rules, migrants who cannot prove they have been in the US continuously for more than two years can be immediately deported without following legal process.
Until now, expedited deportations could only be applied to those detained near the border who had been in the US for less than two weeks.
Kevin McAleenan, acting secretary of Homeland Security, said the change was “a necessary response to the ongoing immigration crisis” and would help to relieve the burden on courts and detention centres.
US Border Patrol says it has made 688,375 apprehensions on the south-west border since October 2018, more than double that of the previous fiscal year.
Previously, only people detained within 100 miles (160km) of the border who had been in the US for less than 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 (DHS) said the new rules would allow it to pursue large numbers of illegal migrants more efficiently.
Rights groups say hundreds of thousands of people could be affected.
The American Civil Liberties Union (ACLU) says it will challenge the policy in court.
The new rule is expected to be implemented with immediate effect after it is published on Tuesday.