A federal judge on Tuesday ordered the Biden administration to lift Trump-era asylum restrictions that have been a cornerstone of border enforcement since the beginning of Covid.

The US district judge, Emmet Sullivan, ruled in Washington that enforcement must end immediately for families and single adults, calling the ban “arbitrary and capricious”. The administration has not applied it to children traveling alone.

Within hours, the justice department asked the judge to let the order take effect on 21 December, giving it five weeks to prepare. Plaintiffs including the American Civil Liberties Union did not oppose the delay.

“This transition period is critical to ensuring that [the Department of Homeland Security] can continue to carry out its mission to secure the nation’s borders and to conduct its border operations in an orderly fashion,” government attorneys wrote.

On Wednesday, Sullivan granted the five-week delay “with great reluctance”, saying it would “enable the government to make preparations to implement” his ruling.

In that 49-page ruling, Sullivan, who was appointed by Bill Clinton, said authorities failed to consider the impact on migrants and possible alternatives. The ruling appears to conflict with another in May by a federal judge in Louisiana that kept the asylum restrictions.

Migrants have been expelled from the US more than 2.4m times since the rule took effect in March 2020, denying migrants rights to seek asylum under US and international law on grounds of preventing the spread of Covid. The practice was authorized under Title 42 of a broader 1944 law covering public health.

Before the judge in Louisiana kept the ban in place in May, US officials said they were planning for as many as 18,000 migrants a day under the most challenging scenario, a staggering number. In May, migrants were stopped an average of 7,800 times a day, the highest of Joe Biden’s presidency.

Immigration advocacy groups have pressed hard to end Title 42, but more moderate Democrats, including senators Mark Kelly of Arizona and Raphael Warnock of Georgia, wanted it to stay when the administration tried to lift it in May.

On Wednesday, Vanessa Cárdenas, executive director of America’s Voice, which advocates for common-sense immigration reform, said: “Keeping Title 42 in place has perpetuated the cruel legacy of the Trump administration and made border enforcement much more difficult and chaotic.

“It’s only fitting that Judge Sullivan’s important ruling came on the same day that Donald Trump announced another run for office and only a week after the American people largely rejected … Republican candidates who took an extreme position on immigration in the midterms.”

Cárdenas said the Biden administration should enact “a functional, orderly and humane set of [immigration] policies that upholds and advances our values and laws”.

Under Title 42, bans have fallen largely on migrants from Guatemala, Honduras and El Salvador – in addition to Mexicans – because Mexico allows them to be returned from the US. Last month, Mexico began accepting Venezuelans expelled from the US under Title 42, causing a sharp drop in Venezuelans seeking asylum at the US border.

Nationalities less likely to be subject to Title 42 have become a growing presence at the border, confident they will be released in the US to pursue their immigration cases. In October, Cubans were the second-largest nationality at the border after Mexicans, followed by Venezuelans and Nicaraguans.

The US homeland security department said it would use the next five weeks to “prepare for an orderly transition to new policies at the border”.

“We continue to work with countries throughout the western hemisphere to take enforcement actions against the smuggling networks that entice migrants to take the dangerous and often deadly journey to our land borders and to address the root causes of irregular migration that are challenging our hemisphere as a whole,” the department said.

An ACLU attorney, Lee Gelernt, said Sullivan’s decision renders the Louisiana ruling moot.

“This is an enormous victory for desperate asylum seekers who have been barred from even getting a hearing because of the misuse of public laws,” Gelernt said. “This ruling hopefully puts an end to this horrendous period in US history in which we abandoned our solemn commitment to provide refuge to those facing persecution.”