The Government is facing fresh pressure to resolve the case of Julian Assange after a United Nations working group decided the WikiLeaks founder is being unlawfully detained while he is holed up in the Ecuadorian embassy in London.
Mr Assange, who has been living in the embassy for more than three years and has been granted political asylum by the Ecuador government, is due to take part in a news conference after the UN panel's decision is formally announced.
Mr Assange believes he will be transported to the United States to be quizzed over the activities of WikiLeaks if he is extradited 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.
Sweden's foreign ministry confirmed the UN panel will rule that Mr Assange has been the victim of "arbitrary" detention inside the embassy in the Knightsbridge area of London.
It is expected the panel will say that the key elements of its decision include the inability of Mr Assange to access the benefit of political asylum, the fact he has never been charged and the length of time he has been inside the embassy.
The group of legal experts has made previous rulings on whether imprisonment or detention is lawful, which have led to people being released but Swedish prosecutors said the ruling will have no impact on its investigation, while the UK Government will argue that it is an opinion rather than a legally binding decision.
Mr Assange's legal team is due to give a news conference in central London shortly after the UN decision is confirmed.
They say the WikiLeaks founder would like to attend the conference in person, but it is more likely he will appear via a video link unless there is a sudden change of heart by the police and the UK Government.
Mr Assange had said he would hand himself over to police for arrest on Friday if the UN group ruled that he had not been unlawfully detained.
The Metropolitan Police have said they will make "every effort" to arrest the WikiLeaks founder should he leave the embassy.
Mr Assange is wanted for questioning in Sweden over one allegation of sexual assault, which he has always denied, and is fighting against extradition.
He was granted political asylum by Ecuador and has remained in their embassy since 2012.
Police have ended a 24 hour guard outside the embassy, but the building remains under covert surveillance.
A Metropolitan 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."
A Foreign Office spokesman said: "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 Ecuadorian embassy.
"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."
A Downing Street spokesman declined to comment on leaks of the UN panel's reported finding, saying the Government would not respond until its official publication.
But he added that the findings will not be "legally binding".