The UK’s controversial plan to deport asylum seekers to Rwanda is unlawful, the Court of Appeal ruled on Thursday in a setback for the government as the hard-line law to curb illegal migration is scrutinized by lawmakers.

The Court of Appeal ruled against of the government’s flagship policy that involves flying refugees who arrive in Britain on small boats some 4,000 miles (6,437.4 kilometers) to the central African nation for processing. The result is that the high court decision upholding the Rwanda policy is reversed, the judges said reading the summary.

Asylum seekers, backed by non-governmental organizations, challenged the policy in London condemning the move as authoritarian. A High Court judge backed the government program as lawful in December.

“The result is that the High Court’s decision that Rwanda was a safe third country is reversed and that unless and until the deficiencies in its asylum processes are corrected removal of asylum-seekers to Rwanda will be unlawful,” the judges said in the majority ruling.

The planned flights to Rwanda — which were slated to take off from a military base in Wiltshire — were thwarted in June of last year after a last-minute intervention from the European Court of Human Rights.

“The deficiencies in the asylum system in Rwanda are such that there are substantial grounds for believing that there is a real risk that persons sent to Rwanda will be returned to their home countries where they faced persecution,” the judges said in a majority decision.

Sunak’s contentious Illegal Migration Bill to stop the flow of small boats crossing the English Channel was voted through by politicians in the House of Commons in April and is now under scrutiny by the House of Lords. According to a government estimate earlier this week, the UK would have to spend £169,000 ($214,450) per person to deport the almost 11,000 people who have made the crossing so far this year.

The number of pending asylum application in the UK mounted to 133,607 as of March end. Three of every four pending applications have been awaiting an initial decision for more than six months.

(Updates with details of the court’s ruling in the first two paragraphs.)