Thousands of migrants who entered the US during the Biden administration and used a special app to arrange asylum appointments are being told to leave "immediately".

The roughly 900,000 migrants who entered at the southern border using the app, CBP One, were generally allowed to remain in the US for two years and given "parole" from immigration laws to work in the country legally.

Now, many of them are being informed that their paroles are revoked and that they are subject to prosecution if they remain in the US.

President Donald Trump has long promised to increase deportations from the US. His administration recently renamed the app to CBP Home and is using it for "self-deportations".

In an email seen by the BBC, a migrant was told, "It is time for you to leave the United States".

"If you do not depart the United States immediately, you will be subject to potential law enforcement actions that result in your removal from the United States - unless you have otherwise obtained a lawful basis to remain here," the email adds.

In a statement, the Department of Homeland Security said that "the Biden Administration abused the parole authority to allow millions of illegal aliens into the US, which further fuelled the worst border crisis in US history".

"Cancelling these paroles is a promise kept to the American people to secure our borders and protect national security," the statement added.

It is unclear how many people received the notices, although immigration officials have confirmed that they have been sent to "some" of those paroled into the US.

DHS said the cancellations and push to leave immediately do not apply to migrants in two parole programmes designated for some Ukrainians and Afghans.

Immigration advocates have said that Mexican, Honduran and Salvadoran migrants are among those who received the notices.

The notice also advises migrants that any benefits received as part of their parole into the US, including work authorisation, are cancelled.

"You will be subject to potential criminal prosecution, civil fines, and penalties, and any other lawful options available to the federal government," it says.

Originally launched in 2020, CBP One 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 an often dangerous journey to the US.

In March, however, the app was rebranded as CBP Home.

It now allows undocumented migrants to identify themselves and declare their intention to leave the country.

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".

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 with 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.