The Trump administration is moving to block undocumented immigrants from seeking asylum in the U.S. unless they present themselves at a port of entry, senior administration officials said.

Under the new policy, migrants apprehended illegally crossing the U.S.-Mexico border between ports of entry will not be eligible to seek asylum, said the officials, who spoke to reporters on background because they were not authorized to speak publicly.

“Those who enter the country between ports of entry are knowingly breaking the law,” said one official Thursday. “While immigration laws do afford people various forms of protection, the reality is it’s a violation of federal law to enter the country [in the manner] illegal aliens are.”

In a statement, DHS Secretary Kirstjen Nielsen and Acting Attorney General Matthew G. Whitaker said the policy is designed to curb use of the asylum process by those not qualified for refuge so that the administration can focus on those with legitimate claims.

“Our asylum system is overwhelmed with too many meritless asylum claims from aliens who place a tremendous burden on our resources, preventing us from being able to expeditiously grant asylum to those who truly deserve it,” said Nielsen and Whitaker.

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The policy is prospective, officials said, and will not affect migrants who have already entered the U.S. It will be posted in the Federal Register as an interim final rule on Friday and will be subject to a 60-day public notice-and-comment period.

The policy, which President Donald Trump previewed prior to Tuesday’s midterm elections, is almost certain to be subject to legal challenges and could end up at the Supreme Court. While entering the U.S. without proper documentation is illegal, current law allows anyone in the U.S. to apply for asylum regardless of whether they arrived legally.

“It is illegal to circumvent that by agency or presidential decree,” said Omar Jadwat, director of the American Civil Liberties Union’s immigrants’ rights initiative, in a statement

The Supreme Court has already upheld one of Trump’s high-profile immigration policies — a ban on travelers from countries, largely Muslim ones, considered a national security threat — and is expected to rule on his attempt to end the Deferred Action for Childhood Arrivals program, which protects around 700,000 “Dreamers” from deportation, sometime next year.

Administration officials said Thursday that the plan is legal because of the president’s discretionary authority over homeland security and immigration policy, including decisions regarding who is allowed into the U.S. The officials noted that the Supreme Court, in upholding the travel ban, cited that authority.

Trump spent the midterm campaign’s final stretch fixated on a caravan of thousands of Central Americans heading north — which he dubbed an “invasion” — and ordered the deployment of 5,200 troops to the southern border to assist the Border Patrol and around 2,000 National Guard troops already stationed there. Critics accused the president of using the caravan, which is still hundreds of miles from the border, to stoke fear among voters as they headed to the polls.

Asylum laws currently on the books have frustrated Trump as he seeks to stop an increase in the number of undocumented immigrants, many of whom are Central American mothers and children seeking refuge from gang violence and poverty, arriving at the southern border. Republicans in Congress tried to change the laws this year, but none of their proposals reached Trump’s desk.

Instead, the president ordered the implementation of a “zero-tolerance” policy that resulted in the separation of more than 2,500 migrant children from their parents this spring. But, facing bipartisan opposition, Trump ended the policy in June and ordered his administration to find a way to detain migrants until they can be deported or are granted asylum in the U.S.

Currently, the government is unable to detain migrant children for longer than 20 days and must release them to a parent if possible, even when that parent is also undocumented and detained. Trump says the practice, known as “catch and release,” encourages more migrants to cross the border illegally because know they will be released into the U.S. pending an appearance in immigration court.

The administration in September proposed a new regulation that would allow the government to hold children in custody together with their parents for the duration of their immigration proceedings. Like the policy announced Thursday, the regulation is likely to face legal challenges.

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