A US judge has issued a scathing opinion against the Trump administration after immigration officials admitted to having mistakenly deported a man living in the US to a prison in El Salvador.

The opinion issued by US District Judge Paula Xinis on Sunday reaffirms her earlier ruling that demands Kilmar Abrego Garcia be returned to the US by Monday night.

The judge says Mr Garcia was seized "without any lawful authority" and held "without legal basis", in "direct contravention" of US law. She calls his situation a "grievous error" that "shocks the conscience".

Trump officials argue that they are unable to return him because they lack the authority to remove him from jail in El Salvador.

The Justice Department has asked an appeals court to dismiss Judge Xinis' order. The appeals court is expected to rule by Sunday evening.

Mr Garcia was among 238 Venezuelans and 23 Salvadorans the Trump administration deported to El Salvador's notorious mega-prison, the Center for the Confinement of Terrorism (Cecot). Officials alleged they were gang members and thus subject to deportation.

Mr Garcia was found to have been wrongly removed in part because a US immigration judge had granted him legal protection from deportation in 2019. His lawyer has denied any gang ties and said Mr Garcia has never been charged with a crime.

Mr Garcia's family, including his wife Jennifer Vasquez Sura, a US citizen, have been calling for his release since his deportation in mid-March.

Judge Xinis' 22-page opinion chastises US officials for claiming that there was little they could do to remedy what they have described as an "administrative error" that led to his deportation.

"As defendants acknowledge, they had no legal authority to arrest him, no justification to detain him, and no grounds to send him to El Salvador - let alone deliver him into one of the most dangerous prisons in the Western Hemisphere," Xinis wrote.

"Having confessed grievous error, the defendants now argue that this court lacks the power to hear this case, and they lack the power to order Abrego Garcia's return."

She added that the government's arguments "fail as a matter of law".

"Surely, Defendants do not mean to suggest that they have wholesale erased the substantive and procedural protections of [federal immigration law] in one fell swoop by dropping those individuals in Cecot without recourse," Xinis wrote.

White House officials continued to defend the deportation. Attorney General Pam Bondi appeared on Fox News on Sunday and reiterated the government's claim that Mr Garcia is a member of the MS-13 gang, which has been designated a terrorist organisation by the Trump administration.

Bondi did not supply any evidence.

Trump also appeared to comment on the case, during his flight back to Washington DC from Florida on Sunday evening.

"He's in MS-13," Trump told reporters aboard Air Force One, without mentioning Mr Garcia by name.

Trump also confirmed that El Salvador President Nayib Bukele will visit the White House in the next few weeks, and that the two leaders will discuss the possibility that the Central American country will begin jailing imprisoned American citizens.

"I love that," he said about the idea.

"I think if we could get El Salvador or somebody to take them, I'd be very happy with it, but I have to see what the law says," he continued.

Xinis' opinion comes a day after the Justice Department lawyer who argued the case on behalf of the Trump administration was put on leave.

Prosecutor Erez Reuvani had told the judge during a hearing on Friday that he was unaware of certain elements of the case, including why Mr Garcia was taken to Cecot prison.

Bondi, in her Fox News appearance, said that Mr Reuvani was removed from the case because he did not sufficiently argue the government's position.

"He shouldn't have taken the case. He shouldn't have argued it, if that's what he was going to do," she said. "You have to vigorously argue on behalf of your client."