The leaked report reveals the treatment of detainees at Guantanamo Bay violates international law and in some cases constitutes torture, according to the Los Angeles Times.
The study urges the US to close the controversial detention centre and bring captives to trial on American territory.
It is the culmination of an 18-month investigation by five UN envoys who interviewed former prisoners, detainees, lawyers and families.
The report is said to conclude that some of the treatment of detainees meets the definition of torture under the UN Convention Against Torture.
The newspaper said the study, ordered by the UN Commission on Human Rights, will be officially published at the end of the week.
The US is holding about 500 people described as "enemy combatants" who were captured in the September 11th attacks on America.
Many are being held on suspicion of having links to al-Qaeda or Afghanistan's ousted Taliban government.
David Hicks, the only Australian detained at the camp, has faced continued delays because of US Federal Court proceedings of another accused detainee, Salim Hamdan, who is challenging the constitutionality of the military commissions.
Hicks' case has been slated for trial by military commission but his American military lawyer, Major Michael Mori, said despite some changes, the process remains unworkable.
The Hicks case is unlikely to be heard before June, which means he would have spent 4 and-a-half years in prison, including 12 months in solitary confinement, before being tried.
US rejects report
The US State Department said the UN report lacks credibility because it is based on hearsay.
Spokesman Sean McCormack claimed the administration had offered the UN delegation an opportunity to visit Guantanamo Bay.
"Sadly, they didn't take us up on the offer. And now they're producing a report based on not having been there," Mr McCormack said.
The delegation turned down the invitation because it would not have been given access to prisoners.