While Trump has not officially made the announcement, Vice President-elect JD Vance confirmed it in a post on X congratulating him on the appointment.

He said, "This is another fantastic pick by the President."

Miller was a policy adviser and speechwriter for Trump in his first term and resumed the function during his campaign, often travelling with Trump to his rallies.

Trump announced last week that he is appointing Susan Wiles, one of his main campaign managers as his Chief of Staff.

On Sunday night, he said that he was appointing Tom Homan as the "Border Czar" overseeing deportations and tightening border security.

Miller and Homan are expected to work closely on the crackdown on illegal migrants that Trump made the centrepiece of his campaign.

Their appointments being among the first is also a measure of the importance Trump places on immigration.

In media interviews, Miller has said that illegal migrants should be rounded up and detained in camps till they are deported.

"Trump will unleash the vast arsenal of federal powers to implement the most spectacular migration crackdown," he warned in an interview with the New York Times last year.

The deportations will "begin on Inauguration Day, as soon as he takes the oath of office," he told Fox News last week.

One of Miller's controversial proposals is ending birthright citizenship - conferring citizenship on anyone born in the US regardless of their parents' status - which Trump has picked up.

His position does not require confirmation by the Senate.

Miller had worked for conservative Republican Congresswoman Michele Bachman and for Jeff Sessions when he was the Attorney General before joining Trump's 2016 campaign.

On Monday morning, Trump said he was naming Congresswoman Elise Stefanik as the permanent representative to the United Nations, the second Cabinet position he has announced after Wiles who has that rank.