A federal judge gave the Trump administration until the end of Monday to return to the United States a Maryland man who was inadvertently deported to El Salvador last month despite a court order allowing him stay in the country.

In a brief ruling issued on Friday, the judge, Paula Xinis, said that federal officials had acted without “legal basis” last month when they arrested the man, Kilmar Armando Abrego Garcia, and put him on a plane — with no due process — to a notorious Salvadoran megaprison.

The decision by Judge Xinis, which came during a hearing in Federal District Court, was a sharp rebuke to the Trump administration. In court papers filed this week, administration officials had said there was little they could do to get Mr. Abrego Garcia back from El Salvador, even after acknowledging that his deportation on March 15 had been a mistake.

The judge’s ruling could put her on a collision course with the White House. President Trump and some of his top aides have repeatedly and aggressively attacked other federal judges who have questioned their attempts to carry out their deportation policies.

Hours after the hearing on Friday, the Justice Department said that it intended to appeal the decision.

The case of Mr. Abrego Garcia, a Salvadoran migrant whose wife and three children are U.S. citizens, has become the latest flashpoint in a multifront battle between immigration lawyers and the White House, which has been ramping up deportations using both traditional and unusual methods.