The US Supreme Court has ruled unanimously that a Maryland man who was mistakenly sent to a mega-jail in El Salvador must be returned to the US.

In a 9-0 ruling, the justices declined to block a lower court’s order to “facilitate and effectuate” bringing back Kilmar Abrego Garcia, but they also said a judge in the case may have exceeded her authority.

The Trump administration had appealed against the district court’s order, but now government lawyers must explain to the judge what steps they are taking to return Mr Garcia to Maryland.

The government has conceded he was deported due to an “administrative error”, though it also alleges he is a member of the MS-13 gang, which his lawyer denies.

Following the Supreme Court’s order, Judge Paula Xinis of the Maryland district court said she wanted to hear by Friday morning how the Trump administration will return Mr Garcia to the US.

The government filed a motion asking for the deadline to be extended until Tuesday evening.

A Maryland court clerk, however, found that the justice department attorneys who filed that motion are not even registered to practise law in the state of Maryland, according to the BBC’s US partner CBS.

It is unclear how this administrative glitch might affect a court hearing in the case that is due to take place before Judge Xinis on Friday afternoon.

In court documents, Mr Garcia’s lawyers accused the government of trying to “delay, obfuscate and flout court orders, while a man’s life and safety is at risk”.

Mr Garcia, a Salvadoran, is one of dozens of alleged gang member migrants placed by the US on military planes last month and flown to El Salvador’s notorious Cecot (Terrorism Confinement Centre) under an arrangement between the two countries.

In a statement on Thursday evening after the top court’s decision, Simon Sandoval-Moshenberg, a lawyer for Mr Garcia, said “the rule of law prevailed”.

“The Supreme Court upheld the district judge’s order that the government has to bring Kilmar home.”

In its emergency appeal to the Supreme Court last week, the Trump administration argued that Judge Xinis lacked the authority to issue the order to return Mr Garcia by Monday night, and that US officials could not compel El Salvador to return him.

US Solicitor General Dean John Sauer wrote in his emergency court filing: “The Constitution charges the president, not federal district courts, with the conduct of foreign diplomacy and protecting the nation against foreign terrorists, including by effectuating their removal.”

On Monday, the Supreme Court, which has a 6-3 conservative majority, temporarily blocked Judge Xinis’s order while it considered the matter.

Issuing its decision in an unsigned order on Thursday, the justices directed Judge Xinis to explain her initial ruling and the extent to which she required the Trump administration to “effectuate” Mr Garcia’s return.

They said she may have exceeded her authority.

“The district court should clarify its directive, with due regard for the deference owed to the executive branch in the conduct of foreign affairs,” the Supreme Court said.

A justice department spokesperson told the BBC that the Supreme Court correctly recognised “it is the exclusive prerogative of the president to conduct foreign affairs”.

“By directly noting the deference owed to the executive branch, this ruling once again illustrates that activist judges do not have the jurisdiction to seize control of the president’s authority to conduct foreign policy.”

The justices did not give the administration a deadline for when Mr Garcia should be returned.

Mr Garcia, 29, entered the US illegally as a teenager from El Salvador. In 2019, he was arrested with three other men in Maryland and detained by federal immigration authorities.

But an immigration judge granted him protection from deportation on the grounds that he might be at risk of persecution from local gangs in his home country.

His US citizen wife, Jennifer Vasquez Sura, has been calling for his release since his deportation.

“I will continue fighting until my husband is home,” she told the New York Times on Thursday.