Having initially issued its opinion in favour of Assange in February, the Working Group on Arbitrary Detention said it was not changing course as the British request "did not meet the threshold of a review... and (was) thus not admissible".
Assange, 45, has been at the Ecuadoran embassy in London since 2012, having taken refuge to avoid being sent to Sweden where he faces rape allegations that he denies.
He fears Sweden would extradite him to the United States to answer for the leaking of diplomatic cables and other classified documents by his whistleblowing website. The disclosures caused huge embarrassment in Washington.
The UN panel, which is attached to the Human Rights Council, met between November 21-25 but only published its findings on Wednesday.
Assange said in a statement: "Now that all appeals are exhausted I expect that the UK and Sweden will comply with their international obligations and set me free. It is an obvious and grotesque injustice to detain someone for six years who hasn't even been charged with an offence."
The fate of the former computer hacker, who turned WikiLeaks into a vehicle for releasing classified documents on the US-led wars in Iraq and Afghanistan, remains unclear.
He was grilled over the longstanding rape allegation by an Ecuadoran prosecutor at the embassy for two days earlier last month. The questions were provided by Swedish officials but the answers were confidential.
Swedish prosecutors dropped a sexual assault probe into Assange last year after the five-year statute of limitations expired. But they still want to question him about the 2010 rape allegation, which carries a 10-year statute of limitations.
Assange insists the sexual encounters in question were consensual.