Mr Assange took refuge in London’s Ecuadorian embassy in 2012 to avoid extradition to Sweden over sex assault claims, which he denies.
He earlier tweeted he would accept arrest if the panel ruled against him, but called for the arrest warrant to be dropped if the decision went his way.
The Met Police said he will still be held if he does leave the embassy.
A warrant for his arrest remains in place.
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 into the case on Friday.
Its panel of legal experts, which has taken evidence from the UK and Sweden, has made previous rulings on whether imprisonment or detention is lawful.
It does not have any formal influence over the British and Swedish authorities and the UK Foreign Office said it still had an obligation to extradite Mr Assange.
Australian Mr 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.
His Wikileaks organisation posted secret American government documents on the internet, and Mr 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.
“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.”
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