TIJUANA, MEXICO - NOVEMBER 24: Members of the 'migrant caravan' wait in line to receive breakfast outside a temporary shelter set up for members of the caravan on November 24, 2018 in Tijuana, Mexico. Around 6,000 migrants from Central America have arrived in the city with the mayor of Tijuana declaring the situation a 'humanitarian crisis'. Most migrants in the caravan say they plan to petition for asylum in the U.S. The incoming government in Mexico will reportedly support a new Trump administration policy requiring migrants asking for asylum in the U.S. to remain in Mexico while their cases are processed.  (Photo by Mario Tama/Getty Images)
Washington CNN  — 

A federal judge in California blocked the Trump administration’s policy of returning some asylum seekers to Mexico to await their immigration court hearing, delivering a major blow to the administration just a day after the Department of Homeland Security Secretary Kirstjen Nielsen was forced to resign.

The preliminary injunction from Richard Seeborg of the Northern District of California is slated to go into effect nationwide on April 12.

The Migrant Protection Protocols program, informally known as “Remain in Mexico,” was initially rolled out at the San Ysidro port of entry in January. Nielsen recently ordered that the policy be expanded along the southern border in an attempt to stem the flow of migrants to the border. Up to 400 people have been returned to Mexico under the policy, according to a DHS official.