Over 350,000 migrants could cross the U.S. southern border in October if a COVID restriction is lifted on Thursday, two Department of Homeland Security officials told NBC News.

The restriction, known as Title 42, was implemented by the Trump administration in March 2020 in response to COVID-19 and it blocked asylum seekers from crossing the U.S.-Mexico border. On Sept. 16, U.S. District Judge Emmet Sullivan ruled that the Biden administration did not have the authority to keep blocking those seeking asylum after facing a lawsuit by advocacy groups. The order could come into effect at some point on Thursday.

"Given that the administration was already this summer allowing most families to seek asylum and has now been in power for eight months, there's no reason why there needs to be any further delay ending Title 42," Lee Gelernt, lead counsel in the litigation for the American Civil Liberties Union who spearheaded the block of Title 42, told NBC.

Secretary Alejandro Mayorkas warned the DHS to prepare for “a worst-case scenario in which 350,000 to 400,000 migrants cross the border,” a number he predicts they might see after Title 42 is lifted but that is not based on specific intelligence calculations.

The Biden administration has appealed Sullivan’s ruling to the U.S. Circuit Court of Appeals and may also appeal to the Supreme Court. This is a move Biden took to address the migration crisis from Central America during the pandemic and turn away adults and some families apprehended at the U.S. southern border, CNN reported.

"Title 42 is not an immigration authority, but a public health authority," Mayorkas said. "To protect the American public. To protect the communities along the border. And to protect the migrants themselves."

However, activists who challenged Biden's order to keep Title 42 do not agree with his intentions behind implementing the order.

"This decision validates what we already know: Title 42 is an illegal and immoral policy based on xenophobia, not science. The court's ruling today makes it clear that the expulsions of vulnerable families into harm's way will not stand," Noah Gottschalk, Oxfam America's global policy lead, said in a statement.

Title 42 applies to all nationalities but the Biden administration has made an exception for children who arrived unaccompanied and has allowed for parents and children arriving together to ask for asylum, NPR noted.

On Wednesday, U.S. Customs and Border Protection reported that authorities detained 208,887 migrants illegally crossing the southern border in August.