Britain's Illegal Migration Bill, which aims to stop asylum seekers and migrants from entering the country, contradicts London's obligations under international law, the United Nations said.
British Prime Minister Rishi Sunak's much-debated plan to make it easier to send asylum seekers to Rwanda is set to become law after attempts by the upper house of parliament to amend the legislation failed.
The bill "is at variance with the country's obligations under international human rights and refugee law and will have profound consequences for people in need of international protection", the UN refugee and human rights chiefs said on Tuesday.
In a joint statement, they said it would deny asylum to anyone trying to cross into the UK from a country where they are not being persecuted.
The bill was passed by parliament. Now it is expected to be approved by King Charles III and become law. The entry into force of the law means that migrants arriving by boat will be denied the right to apply for asylum in the UK.
UN human rights chief Volker Turk and UN refugee chief Filippo Grandi said the bill prevents migrants from submitting protection claims regardless of their asylum status and grants sweeping detention, although judicial review is limited.
"This new legislation significantly erodes the legal framework that has protected so many," Grandi said, adding that it puts refugees at serious risk in contravention of international law.
"I urge the UK government to renew this commitment to human rights by reversing this law and ensuring that the rights of all migrants, refugees and asylum-seekers are respected, protected and fulfilled, without discrimination," Turk said.
South Korea's Dong-a Ilbo daily, citing South Korea's army, identified the person as Travis King, a US.army soldier with the rank of private second class. The newspaper later deleted the name.