The app, known as CBP Home, allows migrants to submit an “intent to depart”, which US Customs and Border Patrol says offers them a chance to leave without “harsher consequences”.

US officials have repeatedly suggested that undocumented migrants in the country should leave voluntarily, rather than be arrested and subject to deportation.

This is the latest move in the White House’s effort to dramatically overhaul the US immigration system, which has included promises of mass detentions.

Originally launched as CBP One in 2020, the mobile application was expanded during the Biden administration to allow prospective migrants to book appointments to appear at a port of entry.

At the time, officials credited the application with helping reduce detentions at the border and portrayed the technology as part of a larger effort to protect asylum seekers making the often dangerous journey.

Now, on the newly rebranded application, undocumented migrants identify themselves and declare their intention to leave the country.

In a statement, Homeland Security Secretary Kristi Noem said that by self-deporting through the app, migrants “may still have the opportunity to return legally in the future and live the American dream”.

“If they don’t, we will find them, we will deport them, and they will never return,” she added.

The app also asks migrants whether they have “enough money to depart the United States” and whether they have a “valid, unexpired passport from your original country of citizenship”.

The BBC has contacted DHS for further details about how the process works once the forms on the app are filled out.

CBP Home can also be used to apply and pay for I-94 entry and exit cards up to seven days before travel, book inspections for perishable cargo and check wait times at US border crossings.

According to DHS, the app is meant to complement a $200m (£155m) domestic and international ad campaign calling for undocumented migrants to “stay out and leave now”.

The Trump administration moved quickly to scrap the CBP One app as part of a larger shift in immigration strategy. It also paused parole programmes, and an uptick in Immigrations and Customs Enforcement (ICE) raids in the country followed.

In late February, the administration said it would create a national registry for undocumented migrants and those failing to sign up could possibly face criminal prosecution.

The registration requires any undocumented migrants above the age of 14 to provide the US government an address and their fingerprints.

Experts said that the registration system will face hurdles, as it is difficult to enforce and fraught with logistical challenges.

Trump on Saturday proclaimed immigrants belonging to the Venezuelan crime gang Tren de Aragua were “conducting irregular warfare” against the US and that he would deport them under the Alien Enemies Act of 1798.

But US District Judge James Boasberg on Saturday evening ordered a halt to deportations covered by the proclamation that will last for 14 days, according to media reports.

Judge Boasberg told a hearing he had heard planes with deportees were taking off and ordered them turned back, the Washington Post reported.

The law allows the US during wartime to detain and remove people threatening the country’s safety without having to follow due process. It was last invoked to intern people of Japanese descent during World War Two.

There was little surprise to the proclamation on Saturday, where Trump declared Tren de Aragua was “perpetrating, attempting, and threatening an invasion or predatory incursion against the territory of the United States”.

He had promised to use the controversial law for mass deportations during last year’s campaign.

The American Civil Liberties Union and other rights group had already sued to block him from using it on Saturday before he issued the proclamation, as well.

At a hearing, the judge said the terms “invasion” and “predatory incursion” in the law “really relate to hostile acts perpetrated by enemy nations,” and the law probably did not offer a good basis for Trump’s proclamation, according to the New York Times.

An ACLU lawyer had told the New York Times he believed there were two planes of Venezuelan immigrants in the air on Sunday. The BBC has not verified that report.

The case will now move through the legal system and could go all the way to the Supreme Court.

The proclamation, and the fight around it, should rally Trump’s supporters, who largely returned him to the White House on his pledges to crack down on illegal immigration and bring down prices of everyday goods. Since he was inaugurated in January, he has swiftly worked to overhaul the US immigration system.

Rights groups, along with some legal experts, are calling the invocation unprecedented, noting the Alien Enemies Act has been used in the past after the US officially declared war against other countries. Under the constitution, only Congress can declare war.

All Venezuelan citizens in the US who are at least 14 years old, members of Tren de Aragua and “are not actually naturalized or lawful permanent residents” were to be “apprehended, restrained, secured, and removed as Alien Enemies”, under Trump’s order.

Trump did not lay out in the proclamation how US officials would determine that a person is a member of the violent, transnational gang.

By using this law, instead of immigration laws that already give him “ample authority” to deport the gang’s members, Trump would not have to prove that detainees are part of Tren de Aragua, said Katherine Yon Ebright, counsel at the Brennan Center for Justice in a statement.

“He wants to bypass any need to provide evidence or to convince a judge that someone is actually a gang member before deporting them,” she said.

“The only reason to invoke such a power is to try to enable sweeping detentions and deportations of Venezuelans based on their ancestry, not on any gang activity that could be proved in immigration proceedings.”