The UN refugee agency on Friday accused Britain of dishonesty over London's plan to send asylum-seekers to Rwanda, as a court heard an emergency bid to block the first deportations next week.
The government intends to fly the first planeload of claimants to Rwanda on June 14, after agreeing the plan with Kigali in a bid to deter illegal migrants from undertaking perilous crossings of the Channel by boat.
Refugee rights groups and a trade union representing UK Border Force personnel are challenging the plan in London's High Court, seeking an injunction against the flight on Tuesday and any beyond then.
They argue that the plan violates asylum seekers' human rights, and say the government cannot justify its claim that Rwanda is a safe destination.
Lawyers for the claimants said that Home Secretary Priti Patel's interior ministry had even claimed endorsement for the plan from the United Nations High Commissioner for Refugees (UNHCR).
But the UN agency's lawyer Laura Dubinsky said it "in no way endorses the UK-Rwandan arrangement".
"UNHCR is not involved in the UK-Rwanda arrangement, despite assertions to the contrary made by the secretary of state," she told the court.
Dubinsky said the would-be refugees were at risk of "serious, irreparable harm" if sent to Rwanda, and that the UN had "serious concerns about Rwandan capacity".
The UNHCR's concerns include a lack of legal redress in Rwanda and potential discrimination against gay claimants.
"These are concerns that have been communicated to the UK authorities and yet the secretary of state's position... is that the UNHCR has given this plan a green light," the claimants' lawyer Raza Husain said.
"That is a false claim."
Lawyers for Patel's Home Office were due to address the court later Friday, with a verdict expected at the end of the day.
The government remains confident it will defeat the challenge, Prime Minister Boris Johnson's spokesman told reporters.
The plan remained "the right approach, not least to tackle the criminal gangs who exploit migrants on the coast of France and quite often force them into unseaworthy vessels to make what is an incredibly dangerous crossing to the UK", he said.
More than 10,000 migrants have made the journey so far this year, a huge increase on prior years. The one-way flights are intended to deter others from entering Britain by illegal routes, and offer those who do try a new life in Rwanda instead.