The UN special rapporteur on torture has formally accused the US government of cruel, inhuman and degrading treatment towards Bradley Manning, the US soldier who was held in solitary confinement for almost a year on suspicion of being the WikiLeaks source.

Juan Mendez has completed a 14-month investigation into the treatment of Manning since the soldier's arrest at a US military base in May 2010. He concludes that the US military was at least culpable of cruel and inhumane treatment in keeping Manning locked up alone for 23 hours a day over an 11-month period in conditions that he also found might have constituted torture.

"The special rapporteur concludes that imposing seriously punitive conditions of detention on someone who has not been found guilty of any crime is a violation of his right to physical and psychological integrity as well as of his presumption of innocence," Mendez writes.

The findings of cruel and inhuman treatment are published as an addendum to the special rapporteur's report to the UN general assembly on the promotion and protection of human rights. They are likely to reignite criticism of the US government's harsh treatment of Manning ahead of his court martial later this year.

Manning, 24, was arrested on May 29 2010 at the Forward Operating Base Hammer outside Baghdad, where he was working as an intelligence analyst. Manning has been charged with 22 counts, including aiding the enemy, relating to the leaking a massive trove of state secrets to the whistleblowing website WikiLeaks.

Mendez, who runs the UN office that investigates incidents of alleged torture around the world, told the Guardian: "I conclude that the 11 months under conditions of solitary confinement (regardless of the name given to his regime by the prison authorities) constitutes at a minimum cruel, inhuman and degrading treatment in violation of article 16 of the convention against torture. If the effects in regards to pain and suffering inflicted on Manning were more severe, they could constitute torture."

Manning was initially held for almost three months at Camp Arifjan in Kuwait, and then transferred in July 2010 to the Marine corps base at Quantico in Virginia. He was held there for another eight months in conditions that aroused widespread condemnation, including being held in solitary confinement for 23 hours a day and being made to strip naked at night.

In his opening letter to the US government on December 30 2010, Mendez said that the prolonged period of isolated confinment was believed to have been imposed "in an effort to coerce him into 'cooperation' with the authorities, allegedly for the purpose of persuading him to implicate others."

It is known that the US department of justice is conducting a grand jury in Virginia exploring the possibility of bringing charges against Julian Assange, the WikiLeaks founder.

The US mission to the UN in Geneva responded to Mendez on January 27 2011. It said that the US government "is committed to protecting human rights in our country and abroad, and we value the work of the special rapporteur".

In a later letter, dated May 19 2011, the Pentagon's legal counsel told Mendez that it was satisfied that Manning's treatment at Quantico had been fine. "Though Private Manning was classified as a maximum custody detainee at Quantico, he occupied the very same type of single-occupancy cell that all other pretrial detainees occupied."

But the Pentagon's arguments did not impress the special rapporteur. He stressed in his final conclusions that "solitary confinement is a harsh measure which may cause serious psychological and physiological adverse effects on individuals regardless of their specific conditions." Moreover, "[d]epending on the specific reason for its application, conditions, length, effects and other circumstances, solitary confinement can amount to a breach of article seven of the international covenant on civil and political rights, and to an act defined in article one or article 16 of the convention against torture."

He also said that the US government had tried to justify Manning's solitary confinement by calling it "prevention of harm watch". Yet the military had offered no details as to what actual harm was being prevented.

Mendez told the Guardian that he could not reach a definitive conclusion on whether Manning had been tortured because he has consistently been denied permission by the US military to interview the prisoner under acceptable circumstances.

The Pentagon has refused to allow Mendez to see Manning in private, insisting that all conversations must be monitored. "You should have no expectation of privacy in your communications with Private Manning," the Pentagon wrote.

The lack of privacy is a violation of human rights procedures, the UN says, and considered unacceptable by the UN special rapporteur.

Manning's travails in solitary confinement came to an end on April 20 2011 when he was transferred from Quantico to Fort Leavenworth in Kansas, where he was held in more open conditions. He is currently being held in a facility in Virginia so that he can make frequent pre-trial appearances at Fort Meade in Maryland ahead of his eventual court martial.