The United States announced it was withdrawing from the United Nations Human Rights Council on Tuesday, dismissing it as a "cesspool of political bias" for its anti-Israel stance.
It marks the latest rejection of multilateral engagement by the Trump administration - following its exit from the Paris climate accords and the Iran nuclear deal - and comes a day after the UN's most senior human rights official condemned the US for separating children from parents at the border with Mexico.
Mike Pompeo, the secretary of state, and Nikki Haley, US ambassador to the UN, issued the announcement, with Mrs Haley saying the Geneva-based organisation was "not worthy of its name".
Mrs Haley described the council as a "protector of human rights abusers" and accused the body of "politicising and scapegoating countries with positive human rights records".
She said the decision had not been taken lightly, and added: “We take this step because our commitment does not allow us to remain a part of a hypocritical and self-serving organisation that makes a mockery of human rights.”
She said the US would have stayed if the changes they sought had been implemented, and said she did not rule out rejoining at a later date.
Boris Johnson, the Foreign Secretary, said the US decision was "regrettable".
"We’ve made no secret of the fact that the UK wants to see reform of the human rights council, but we are committed to working to strengthen the council from within," he said.
"Britain’s support for the human rights council remains steadfast. It is the best tool the international community has to address impunity in an imperfect world and to advance many of our international goals.
"That’s why we will continue to support and champion it."
Profile | Nikki Haley
The US is halfway through a three-year term and its departure marks the first time that a sitting member would volunteer to step aside.
Libya was suspended in 2011 after a government crackdown on unarmed protesters.
Under President George W Bush the US refused to join the Geneva-based forum when it was created in 2006, but did so after President Barack Obama took office in 2009.
Established to promote human rights worldwide the council has passed more than 70 resolutions critical of Israel, 10 times as often as it has criticised Iran.
The US move came as Donald Trump achieved his highest job rating since the first week of his presidency.
For the first time since January 2017 he recorded an approval figure of 45 per cent in a weekly Gallup poll.
It meant Mr Trump's popularity was exactly the same as Bill Clinton and Ronald Reagan at the same stage of their presidencies, while Barack Obama was at 46 per cent.
The poll was conducted over the course of last week and reflected a strengthening economy, falling unemployment, and the summit with North Korean leader Kim Jong-un in Singapore.
It was unclear how much the poll had been affected by a growing furore over a new "zero tolerance" policy of separating illegal immigrant parents from their children at the Mexican border.
A separate Quinnipiac poll showed 66 per cent of Americans oppose the controversial policy, although 55 Per cent of Republicans support it.
The policy means all illegal immigrants are now detained and prosecuted, and their children are removed and held in separate detention centres.
The UN human rights chief Zeid Ra'ad Al Hussein criticised the policy on Monday, calling it "unconscionable."
Mrs Haley hit back on Tuesday, attacking the council for its own “hypocrisy”.
The council’s current membership includes 14 countries that are ranked as “not free” by Freedom House: Afghanistan, Angola, Burundi, China, Cuba, Congo, Egypt, Ethiopia, Iraq, Qatar, Rwanda, Saudi Arabia, United Arab Emirates and Venezuela.
"Once again, the United Nations shows its hypocrisy by calling out the United States while it ignores the reprehensible human rights records of several members of its own human rights council," she said.
In the past, illegal immigrant families were "caught and released" while they awaited proceedings. Over a six-week period more than 2,000 children have been separated from their parents, and photographs have emerged of minors being held in wire mesh cages.
A secretly recorded tape also emerged of Central American children at a detention centre in Texas crying and pleading for their parents as a guard joked: "We have an orchestra here".
Mr Trump on Tuesday said America was being "infested" by illegal immigrants and members of the MS-13 gang, and blamed Democrats for the crisis.
He wrote on Twitter: "Democrats are the problem. They don’t care about crime and want illegal immigrants, no matter how bad they may be, to pour into and infest our country, like MS-13. They can’t win on their terrible policies, so they view them as potential voters! We must always arrest people coming into our country illegally."
Asked if Theresa May thought it was acceptable to cage children, the prime minster's spokesman said: "The welfare and safeguarding of children is at the heart of our immigration policy. We do not separate child refugees or asylum seekers from their families."
Jeff Sessions, the US attorney general, dismissed comparisons to Nazi concentration camps. He told Fox News: "Well, it's a real exaggeration, of course. In Nazi Germany, they were keeping the Jews from leaving the country."
There was condemnation from all four living US former first ladies, and a host of senior Republicans including Senator John McCain. Ted Cruz, the high profile conservative Republican senator from Texas, also said he was "horrified" and that "this has to stop".
Mr Cruz said he was introducing a bill in Congress to allow illegal immigrant families to stay together in temporary shelters.
Mr Trump was scheduled to meet with Republicans in Congress to discuss potential immigration bills on Tuesday night.
At a meeting for small business owners on Tuesday, he said: "We want to solve this problem. I don't want children taken away from their parents.
"When you prosecute the parents for coming in illegally, which should happen, you have to take the children away."
He said there were only two choices - "totally open borders or criminal prosecution".
Mr Trump said he was asking Congress for a "third option".
Democrats accused the president of using the separation of families as a "negotiating tool" as he sought to strong-arm them into supporting funding for a border wall.
Meanwhile, more than $4.8 million was raised on Facebook to help separated families with legal services.
It was the social network's largest ever fundraiser and those donating included chief executive Mark Zuckerberg.