Free and to be compensated should be allowed for Julian Assange

Mr Assange, 44 - who faces extradition to Sweden over a rape claim, which he denies - claimed asylum in London's Ecuadorean embassy in 2012.

He has been arbitrarily detained since his arrest in 2010, the panel said.

The UK Foreign Office said the report "changes nothing" and it will "formally contest the working group's opinion".

The panel's ruling is not legally binding in the UK and a European Arrest Warrant remains in place - meaning the UK continues to have a legal obligation to extradite Mr Assange.

The UN's Working Group on Arbitrary Detention insisted Mr Assange's detention "should be brought to an end, that his physical integrity and freedom of movement be respected".

"Mr Assange should be afforded the right to compensation," it added.

The Wikileaks founder had been subjected to "different forms of deprivation of liberty" it said, initially while he was held in isolation at London's Wandsworth Prison for 10 days in 2010.

The deprivation had been "continuous" since he was first arrested in the UK on 7 December 2010.

The panel said he had then been under "house arrest and then confinement" while inside the Ecuadorian embassy.

It also found a "lack of diligence" by the Swedish Prosecutor's Office in its investigations, which resulted in his lengthy loss of liberty.

A Foreign Office spokesman said: "Julian Assange has never been arbitrarily detained by the UK.

"The opinion of the UN Working Group ignores the facts and the well-recognised protections of the British legal system. He is, in fact, voluntarily avoiding lawful arrest by choosing to remain in the Ecuadorean embassy."

"We are deeply frustrated that this unacceptable situation is still being allowed to continue," the spokesman added.

The Swedish government said Mr Assange was free to leave the embassy at any point and said he was not being deprived of his liberty.

In September 2014, Mr Assange - who has been living in the embassy for more than three years - complained to the UN that he was being "arbitrarily detained" as he could not leave without being arrested.

The complaint against the UK and Sweden claimed Mr Assange had been deprived of his liberty for an "unacceptable length of time".

In a statement posted on Twitter on Thursday, Mr Assange said his passport should be returned and his arrest warrant dropped if the UN panel ruled in his favour.

'Avoiding lawful arrest'

The Australian was originally arrested in London in 2010 under a European Arrest Warrant issued by Sweden over rape and sexual assault claims.

In 2012, while on bail, he claimed asylum inside the Ecuadorean embassy in Knightsbridge after the UK Supreme Court had ruled the extradition against him could go ahead.

Swedish prosecutors dropped two sex assault claims against Mr Assange last year. However, he still faces the more serious accusation of rape.

In the statement, published by Wikileaks on Thursday, Mr Assange said: "Should the UN 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.

"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."

Last October, Scotland Yard said it would no longer station officers outside the Ecuador embassy following an operation which it said had cost £12.6m. But it said "a number of overt and covert tactics to arrest him" would still be deployed.