Controversial move confirmed by Attorney General Jeff Sessions as Democrats, activists vow to challenge decision.
The United States government is ending a programme that has allowed young immigrants brought to the country illegally as children to remain in the United States, putting at risk the status of hundreds of thousands of immigrants.
US President Donald Trump's administration will wind down the programme known as Deferred Action for Childhood Arrivals (DACA) as participants' authorisations expire, Attorney General Jeff Sessions said Tuesday.
Ten US states had challenged the programme, demanding the government take action by Tuesday or face a lawsuit.
Immigrant groups denounced the decision and warned it could result in the deportation of participants - often known as Dreamers - who are American in all but name, having spent most of their lives in the United States.
"If the president cannot find it in his heart to protect innocent, patriotic young people from being deported to countries they do not know, we cannot expect him to act honourably in times of moral crisis," Cristobal Alex of the Latino Victory Project advocacy group said, calling for legal action and for Republicans in Congress to act.
Sessions determined that the programme, enacted by former president Barack Obama, had illegally bypassed Congress, which must draft immigration legislation.
Sessions called Obama's action - taken after Congress failed to pass immigration reforms - "an unconstitutional exercise of authority by the executive branch" and cast the decision as a return to the rule of law.
"Simply put, if we are to further our goal of strengthening the constitutional order and the rule of law in America, the Department of Justice cannot defend this type of overreach," he said.
He claimed the move had led to a surge in unaccompanied children arriving at the US southern border and denied jobs to American citizens.
"Basic decency". Obama dismissed the argument that there was a legal case and instead calling the decision a matter "about basic decency."
"These Dreamers are Americans in their hearts, in their minds, in every single way but one: on paper," he said.
"To target these young people is wrong – because they have done nothing wrong," said Obama. "It is self-defeating – because they want to start new businesses, staff our labs, serve in our military, and otherwise contribute to the country we love. And it is cruel."
Trump said Congress now needed to address the issue, stressing that he does "not favour punishing children, most of whom are now adults, for the actions of their parents. But we must also recognize that we are nation of opportunity because we are a nation of laws."
He vowed to "resolve the DACA issue with heart and compassion," but stressed it must be done through the democratic process in a way that benefits US citizens.
"We must remember that young Americans have dreams too. Being in government means setting priorities," he said. "Our first and highest priority in advancing immigration reform must be to improve jobs, wages and security for American workers and their families."
White House spokeswoman Sarah Sanders painted the move as a measured decision that allows the programme to be wound down over time and for Congress to act, instead of having millions of people's status changed overnight by a potential court order.
Speaker of the House Paul Ryan, the top Republican in Congress, said he hoped Congress would "find consensus on a permanent legislative solution that includes ensuring that those who have done nothing wrong can still contribute as a valued part of this great country."
"Clock is ticking". Top lawmakers from the US Republican and Democratic parties said they will join together to push legislation later this month.
"The clock is ticking, we are now in a countdown to deportation," said Democratic Senator Dick Durbin, who first introduced legislation on the matter 16 years ago.
Senator Lindsay Graham, a Republican, sought to reassure the immigrants that the Congress could pass legislation and called on Trump to work with lawmakers to achieve that goal.
"We're not going to allow these kids to be victims of a broken political process," Graham said. "This may be what we need, real people ... who need us to act decisively."
US immigration authorities said the programme would be phased out in a way that provided minimal disruption, with officials continuing to process renewals under the programme during the next six months and allowing participants' permits to expire over time through 2019.
Nearly 800,000 people had been approved for the programme, which protects participants from deportation and allows them to work or attend university.
It was unclear what would happen to the immigrants once their DACA participation ends and their work permits expire. Sanders stressed, however, that the group was not a current target of immigration enforcement and would not be in the future.
What is DACA? Former US president Barack Obama implemented a programme in 2012 that allows many immigrants brought to the country illegally as children to remain in the United States.
President Donald Trump's administration chose Tuesday to phase out the programme known as Deferred Action for Childhood Arrivals (DACA).
Obama implemented the programme through an executive order after failed attempts to reform the immigration system through Congress.
Nearly 800,000 people had been approved for the programme that allows participants, sometimes known as Dreamers after the so-called Dream Act that would have provided legal status to young immigrants, to remain in the country and work or attend school.
DACA does not provide a pathway to citizenship, but does grant a two-year reprieve from deportation to those who meet requirements, such as having completed high school or served in the US military. These benefits can be renewed. Applicants must have been under the age of 31 at the time of the programme's implementation.
Immigration authorities said Tuesday they would continue to process renewals for the next six months, but that participants' status would expire at the end of their current authorization under the programme.
The Migration Policy Institute estimates some 1.9 million unauthorized immigrants are potentially eligible for the programme and that 68 per cent of those have applied.
Some one-third or participants had enrolled in college and 76 per cent were working, the institute estimates.
Another proposal to shield millions more illegal immigrants, including the immigrant parents of US-born citizens, from deportation was blocked by the US Supreme Court.
The DACA programme faced an imminent legal challenge from attorneys general in 10 Republican-controlled states if Trump does not act against the programme by Tuesday.