WikiLeaks' Assange 'arbitrarily detained' in Ecuador's London embassy, UN panel to say

* Assange has been holed up in Ecuador embassy since 2012

* Says will leave embassy if UN panel rules against him

* Expects passport back if UN rules in his favour

* Sweden says UN panel to rule in Assange's favour

* Britain to arrest Assange if he leaves the embassy (Adds sourcing for UN panel decision, changes wording of headline)

By Johan Ahlander and Guy Faulconbridge

STOCKHOLM/LONDON, Feb 4 (Reuters) - 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 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.

He denies allegations of rape in 2010 and has portrayed them as a ploy to have him eventually sent to the United States, 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 U.N. 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 (Xetra: A1W6XZ - news) 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 United States and its allies by using his WikiLeaks website to leak hundreds of thousands of secret U.S (Other OTC: UBGXF - news) . diplomatic and military cables in 2010, disclosures that often embarrassed Washington.

He made international headlines in early 2010 when WikiLeaks published classified U.S. military video showing a 2007 attack by Apache (NYSE: APA - news) helicopters that killed a dozen people in Baghdad, including two Reuters news staff.

Later that year, the group released over 90,000 secret documents detailing the U.S.-led military campaign in Afghanistan, followed by almost 400,000 internal U.S. military reports detailing operations in Iraq.

Those disclosures were followed by the release of more than 250,000 classified cables from U.S. embassies. It (Other OTC: ITGL - news) would go on to add almost three million more diplomatic cables dating back to 1973.

POLITICAL REFUGEE?

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

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.

Per Samuelson, one of Assange's Swedish lawyers, said if the U.N. panel judged Assange's time in the embassy to be custody, he should be released immediately.

"It is a very important body that would be then saying that Sweden's actions are inconsistent with the European Convention on Human Rights. And it is international common practice to follow those decisions," Samuelson told Reuters.

Since Assange's confinement, WikiLeaks has continued to publish documents on topics such as the Trans-Pacific Partnership, one of the world's biggest multinational trade deals, which was signed by 12 member nations on Thursday in New (KOSDAQ: 160550.KQ - news) Zealand. (Additional reporting by Matt Siegal in Sydney, Daniel Dickson in Stockholm and Michael Holden and Elizabeth Piper in London. Editing by Jeremy Gaunt.)