The UN Working Group on Arbitrary Detention threw out a UK appeal against its previous ruling, when the panel had said that Mr Assange”s detention was unlawful.
Although Mr Assange voluntarily sought refuge in the Ecuadorian Embassy in London more than four years ago, the panel ruled that the fact that he would be arrested as soon as he attempted to leave amounted to an arbitrary detention.
In throwing out the appeal, the UN said the British Government had not brought forward any new evidence to allow a review of the case.
Following the ruling, Mr 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.”
In a statement, campaign group Justice for Assange said the decision meant that “the UK and Sweden are once again required to immediately put an end to Mr Assange”s arbitrary detention and afford him monetary compensation”.
But it is not clear what sway the verdict will have over the Government, which slammed the decision.
Foreign Office Minister for the Americas, Sir Alan Duncan, said: “Julian Assange is not, and has never been, arbitrarily detained in the UK and his continued presence in the Ecuadorean Embassy is entirely self-inflicted.
“We completely reject the opinion of the UN Working Group and are very disappointed that they will not review their deeply flawed and incorrect position.
“A European Arrest Warrant for an allegation of rape remains outstanding and the UK has a legal obligation to extradite him to Sweden.”
He added: “He has exhausted the well-recognised protections available to him under the British legal system.
“He has been avoiding arrest by choosing to remain in the Ecuadorean Embassy for more than four years now and the UK wants to see a conclusion to this case.
“We continue to encourage Ecuador and Sweden to work together to resolve this frustrating situation and we will continue to work to support that happening.”
Mr Assange has been hiding in the embassy since allegations he raped a woman in Sweden in 2010 emerged.
He claims he will be extradited across the world to the US to face charges over the activities of Wikileaks if he goes to Sweden to face justice.
Last month he gave his first ever interview to Swedish authorities over the allegations from within the embassy.