The principle of not sending a person to a place where he may be persecuted is "beyond doubt" customary international law, the UN refugee agency says.
The United Nations High Commissioner for Refugees weighed in yesterday at the hearing of three African men who were making their last attempt to challenge the way the city vetted refugee claims.
The trio argues that the government must assess the applications itself rather than passing the responsibility to the UNHCR.
"[The principle] is, at minimum, customary international law," Gerard McCoy SC, for the UNHCR, told the Court of Final Appeal. "The matter, we say, is simply beyond doubt."
The UN agency said it seemed that the Hong Kong government wanted to contend Asian countries were somehow exempted, McCoy said.
"This has to be refuted. The idea of a Hong Kong or regional opt-out from this fundamental cornerstone is inconceivable."
The barrister said the UNHCR would reserve the right to argue that the principle was peremptory, that is, one so fundamental that no state may depart from it.
Benjamin Yu SC, for the government, said it could let someone stay by exercising its discretion on humanitarian grounds, but it was not obliged to do so.
Hong Kong is not a signatory to the UN Convention Relating to the Status of Refugees, and is therefore not subject to obligations under it. The government does not take applications for refugee status but instead refers them to the UNHCR.
The three men - in their 20s and 30s from the Democratic Republic of Congo and Republic of Congo - point to problems in the UNHCR's assessment process, including that its decisions are immune from judicial scrutiny.
They also say the principle to which McCoy has referred, known as non-refoulement, has become a norm from which no state can depart.
The UNHCR was not a party to the proceedings until recently. Late last year, against the government's objections, a judge allowed its application to join, McCoy said outside court.
He told the court that the agency operated only under a mandate of protection from the UN. Any immigration decisions, such as whether to deport a person, were for states to make and no country could pass its legal responsibilities to the UNHCR, he said.
McCoy also said the agency's decisions were not susceptible to judicial scrutiny.
All three men, who have not been identified by name, earlier had their applications for refugee status rejected by the UNHCR, as well as their appeals.
The Security Bureau said it would not comment as the legal proceedings were ongoing.
The hearing continues before Mr Justices Patrick Chan Siu-oi, Roberto Ribeiro, Robert Tang Ching, Kemal Bokhary and Anthony Mason.