WikiLeaks founder Julian Assange's three-and-a-half-year stay in the Ecuadorian embassy in London to avoid rape investigation in Sweden amounts to 'arbitrary detention', a United Nations panel will rule on Friday.

Assange, a former computer hacker who has been holed up in the embassy since June 2012, told the UN 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.

He denies allegations of rape in 2010 and has portrayed them as a ploy to have him eventually sent to the US, where he could be put on trial over WikiLeaks' publication of the classified military and diplomatic documents.

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

But the UN panel of outside experts has ruled in Assange's favour, Sweden said.

"(The) working group has made the judgment that Assange has been arbitrarily detained in contravention of international commitments," a spokeswoman for the Swedish Foreign Ministry said, confirming a report by the BBC.

The ruling is to be published on Friday.

Assange had said earlier in a short message on Twitter that he would have left the embassy if the UN panel had ruled against him.

"(But) 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," Assange, 44, said.

Britain said Assange will be arrested if he leaves his cramped quarters at the embassy and then extradited to Sweden.

The decision in his favour marks the latest twist in a tumultuous journey for Assange since he incensed the US and its allies by using his WikiLeaks website to leak hundreds of thousands of secret US diplomatic and military cables in 2010, disclosures that often embarrassed Washington. - Reuters