A UN panel has ruled Wikileaks founder Julian Assange is being “arbitrarily detained”.
Assange claimed asylum in London’s Ecuadorean embassy in 2012 to avoid extradition to Sweden over sex assault claims, which he denies.
The Met Police said he will still be held if he does leave the embassy.
He earlier tweeted he would accept arrest if the panel ruled against him, but called for his arrest warrant to be dropped if the decision went his way.
In 2014, Mr Assange complained to the UN that he was being “arbitrarily detained” as he could not leave the embassy without being arrested.
The UN’s Working Group on Arbitrary Detention is due to announce the findings of its investigation on Friday.
While the BBC understands the panel will find in Mr Assange’s favour, Wikileaks tweeted it was waiting for “official confirmation”.
The UK government said it would not “pre-empt” the ruling, saying Assange still faced one allegation of rape while a European Arrest Warrant remained in place.
“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 spokesman added.
“The UK continues to have a legal obligation to extradite Mr Assange to Sweden.”
The panel’s decision is not legally binding on the UK or Sweden, Clive Coleman, BBC legal affairs correspondent said.
Assange will argue the decision is significant and adds considerable legal and moral force to the argument he is being arbitrarily detained, he said.
But our correspondent added the UK government is likely to argue that Mr Assange’s detention follows “an entirely lawful process”.
Australian Assange was originally arrested in London in 2010 under a European Arrest Warrant issued by Sweden.
He claimed asylum inside the Ecuadorian embassy in Knightsbridge after the UK Supreme Court 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.
His Wikileaks organisation posted secret American government documents on the internet, and Assange says he believes Washington will seek his transfer to the US if he is sent to Sweden.
In the statement, published earlier by Wikileaks on Twitter, 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.
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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.”
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.
Assange has been holed up in the diplomatic mission in west London since 2012 and has been granted political asylum by the Ecuador government.
He is wanted for questioning in Sweden over allegations of sexual assault against two women, which he has always denied.
He claims if he went to Sweden he could be handed over the US authorities for prosecution over WikiLeaks disclosures.
The UN working group has no power to force the UK and Sweden to abide by its decision, but it has been influential in winning the freedom of several high profile international detainees including Myanmar’s Aung San Suu Kyi.
A Met Police spokesman said: “The operation to arrest Julian Assange does however continue and should he leave the Embassy the MPS will make every effort to arrest him.”
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