GENEVA — A United Nations expert on torture said on Friday that an examination of the WikiLeaks founder Julian Assange in a British prison showed an alarming deterioration in his mental and physical state, and he sharply rebuked Britain, Sweden and the United States for “ganging up” on Mr. Assange.

The United Nations special rapporteur on torture and ill treatment, Nils Melzer, said the examination in early May revealed that Mr. Assange’s “capacity to focus and coordinate have been clearly affected” by his imprisonment.

“He was extremely jumpy and stressed,” Mr. Melzer said in an interview. “It’s difficult to have a structured conversation with him. There’s so much going on in his mind it’s difficult to have a dialogue with him.”

Furthermore, he said in a statement, Mr. Assange should not be extradited the United States, where he faces charges of conspiracy to hack into a Pentagon computer. He said that the cumulative effects of Mr. Assange’s punishment can only be described as “psychological torture.”

Mr. Assange, 47, hailed by many as a champion of transparency, has been sought by prosecutors around the globe on a variety of charges. He jumped bail in Britain and sought refuge in the Ecuadorean Embassy in London as Sweden was in the middle of investigating a rape accusation against him. The United States has accused him of helping an Army private to illegally download and leak classified information in 2010, much of it about the wars in Iraq and Afghanistan.

[Update: Sweden has dropped the inquiry into rape allegations against Julian Assange.]

After he lived in the Ecuadorean Embassy in the British capital for seven years, his relationship with his hosts soured over his arguments with the embassy’s staff, his skateboarding indoors and what the nation’s officials described as threats and leaks meant to embarrass Ecuador. The country’s vice president, who blamed WikiLeaks, called the actions “despicable” and vowed to take action.

In April, the Ecuadorean government — which had granted him asylum and, eventually, citizenship — withdrew its protection. The British police then hauled Mr. Assange out of the embassy and placed him under arrest. A British court later sentenced him to 50 weeks in jail for violating the terms of his bail.

On Thursday, Mr. Assange was to appear by video link from Belmarsh Prison in East London at a court hearing on the United States’ application for his extradition. But his British lawyer, Gareth Peirce, said her client was not well enough to participate. The presiding magistrate set June 12 for the next hearing and suggested it could be held in Belmarsh.

WikiLeaks said on Thursday that Belmarsh Prison authorities had moved Mr. Assange to its hospital wing after he experienced drastic weight loss and other health problems. It also said that when Mr. Assange’s Swedish lawyer, Per Samuelson, visited him in Belmarsh on Friday, he found “that it was not possible to hold a normal conversation with him.”

Mr. Melzer is not the first United Nations expert to criticize the conditions of Mr. Assange’s detention. The United Nations Working Group on Arbitrary Detention condemned the 50-week sentence for jumping bail as excessive and said that sending him to a high-security prison was akin to a conviction for a more serious crime.

Mr. Melzer said he had initially been skeptical about Mr. Assange’s case and had turned down a request from Mr. Assange’s lawyers in December to investigate. But he said that what he found after having accepted a second request from the lawyers in March, changed his mind.

“Wherever I delved into the case, I found a lot of dirty stuff,” he said in a phone interview.

In 20 years of working with victims of war, violence and political persecution, Mr. Melzer added in his statement, he had “never seen a group of democratic states ganging up to deliberately isolate, demonize and abuse a single individual for such a long time and with so little regard for human dignity and the rule of law.”

Mr. Melzer said that Britain should not extradite Mr. Assange to the United States or to any other country that did not provide reliable guarantees that it would not transfer him to the United States. He cited the treatment experienced by Chelsea Manning, the former Army intelligence analyst who sent WikiLeaks classified cables on events in Iraq and Afghanistan, as grounds for concern about the conditions in which Mr. Assange would be held.

He also said that he was convinced Mr. Assange would not receive a fair trial. He challenged the conduct of Sweden’s prosecutors and criticized Britain’s limitations on Mr. Assange’s access to lawyers and to the complex documents relating to the charges, which he said severely impaired his ability to prepare his defense.

In his statement, Mr. Melzer cited what he called systematic abuse of judicial powers; arbitrary confinement in Ecuador’s London Embassy; harassment and surveillance inside the embassy; and a “relentless and unrestrained campaign of public mobbing, intimidation and defamation” outside it.

But John Demers, the head of the Justice Department’s National Security Division, addressing the leaks of Pentagon records linked to Mr. Assange, condemned him in May, telling reporters: “No responsible actor, journalist or otherwise, would purposefully publish the names of individuals he or she knew to be confidential human sources in a war zone, exposing them to the gravest of dangers.”

During the 2016 presidential race, WikiLeaks also released thousands of emails hacked from the Democratic National Committee and the personal account of John D. Podesta, the chairman of Hillary Clinton’s campaign, in an effort to harm her candidacy. The special counsel, Robert S. Mueller III, concluded that the emails had been stolen by Russian intelligence agents, which Mr. Assange has denied.