WikiLeaks founder Julian Assange's three-and-a-half-year stay in the Ecuadorian embassy in London amounts to 'unlawful detention', a United Nations panel examining his appeal will rule on Friday, the BBC reported.

Assange, a former computer hacker who has been holed up in the embassy since June 2012, told the U.N. Working Group on Arbitrary Detention that he was a political refugee whose rights had been infringed by being unable to take up asylum in Ecuador.

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Reuters was unable immediately to confirm the BBC report, and the UN said the panel's opinion, which is not legally binding, was due to be published on Friday.

Britain said it had never arbitrarily detained Assange and that the Australian had voluntarily avoided arrest by jumping bail to flee to the embassy.

It said Assange will be arrested if he leaves the embassy and then extradited to Sweden for questioning over allegations of rape in 2010. Assange denies the rape allegations.

"Should the U.N. announce tomorrow that I have lost my case against the United Kingdom and Sweden, I shall exit the embassy at noon on Friday to accept arrest by British police as there is no meaningful prospect of further appeal," Assange said in a statement posted on the Wikileaks Twitter account.

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"However, should I prevail and the state parties be found to have acted unlawfully, I expect the immediate return of my passport and the termination of further attempts to arrest me."

A decision in his favor would mark the latest twist in a tumultuous journey for Assange since he incensed the United States and its allies by using his WikiLeaks website to leak hundreds of thousands of secret U.S. diplomatic and military cables in 2010, disclosures that often embarrassed Washington.

Assange, 44, fears Sweden will extradite him to the United States, where he could be put on trial over WikiLeaks' publication of the classified military and diplomatic documents, one of the largest information leaks in U.S. history.

In a submission to the U.N. working group, a body of outside experts, Assange argued that his time in the embassy constituted arbitrary detention.

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Assange says he is the victim of a witch hunt directed by the United States and that his fate is a test case for freedom of expression.

He said that he had been deprived of his fundamental liberties, including lack of access to sunlight or fresh air, adequate medical facilities, as well as legal and procedural insecurity.

"We have been consistently clear that Mr. Assange has never been arbitrarily detained by the UK but is, in fact, voluntarily avoiding lawful arrest by choosing to remain in the Ecuadorean embassy," a British government spokeswoman said.

"An allegation of rape is still outstanding and a European Arrest Warrant in place, so the UK continues to have a legal obligation to extradite Mr. Assange to Sweden," she said.