Mr Assange has been living in the Ecuadorian embassy in London for more than three years and has been granted political asylum by the Ecuador government.
He is wanted for questioning in Sweden over one sex assault allegation, which he has always denied.
He believes he will be taken to the US to be quizzed over the activities of WikiLeaks if he goes to Sweden. There is an espionage case against him in the US.
He filed a complaint against Sweden and the UK in September 2014 which has been considered by the UN Working Group on Arbitrary Detention.
The group is made up of legal experts and has made previous rulings on whether imprisonment or detention is lawful, which have led to people being released.
It is understood a decision will be published on Friday.
If the group concludes that Mr Assange is being unlawfully detained, the UN is expected to call on the UK and Sweden to release him.
Mr Assange has offered to be interviewed by Swedish prosecutors in the embassy .
Before entering the embassy, Mr Assange was under house arrest and in prison since December 2010. He has not been charged.
BREAKING: UN set to announce whether it will order WikiLeaks founder Julian Assange's release on Friday; more https://t.co/yIHlDNzabh— WikiLeaks (@wikileaks) February 2, 2016