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.

Seeborg found that the provision on which the government was relying was never intended to permit the return of asylum seekers to Mexico, and even if it did, the current screening procedures were not adequate, Melissa Crow, senior supervising attorney at the Southern Poverty Law Center, said in a statement.

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