A federal judge in Texas has ruled that a programme protecting migrants who came to the US as children from deportation is illegal.
Judge Andrew Hanen said new applicants should not be enrolled in Deferred Action for Childhood Arrivals (Daca).
But he said the ruling does not require the government to deport any Daca recipients.
Nearly 650,000 people - known as Dreamers - are currently enrolled in the programme.
President Joe Biden has ordered his administration to strengthen the initiative. The US Supreme Court last year blocked a bid by former President Donald Trump to end Daca.
In Friday's ruling, Judge Hanen agreed with a group of states that had filed a lawsuit arguing the programme was illegally created by former President Barack Obama in 2012.
Texas and eight other conservative states said Mr Obama had acted without due congressional authorisation.
Daca recipients are shielded from deportation, granted work authorisation, can have driving licences and apply for education financial aid.
This is the second time a federal court in Texas has dealt a blow to Mr Biden's immigration agenda.
In January, a judge blocked the Democratic president's attempt to impose a 100-day moratorium on deportations.
Border officials caught migrants there 188,829 times in June, the largest number in a single month in more than two decades.
This included over 15,000 children who were travelling alone, said US Customs and Border Protection.
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