Ahead of the ruling, Assange said he would leave the embassy and accept arrest on Friday if a UN panel investigating his case rules against him.
"Should the U.N. 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," Assange said in a statement posted on the Wikileaks Twitter account.
"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."
But British police said they would arrest Assange if he leaves the embassy.
"The warrant is still in place. If he leaves the embassy we will make every effort to arrest him," a spokesman for the British police said in London.
A spokesman for Assange could not immediately be reached for comment.
Assange fears Sweden will extradite him to the United States, where he could be put on trial over WikiLeaks' publication of classified military and diplomatic documents, one of the largest information leaks in U.S. history.
The U.N. Working Group on Arbitrary Detention is considering a request for relief by Assange, who argued in a submission that his time in the embassy constituted arbitrary detention.
Assange argued that he had been deprived of his fundamental liberties, including lack of access to sunlight or fresh air, adequate medical facilities, as well as legal and procedural insecurity.
Assange made international headlines in early 2010 when WikiLeaks published classified U.S. military video showing a 2007 attack by Apache helicopters that killed a dozen people in Baghdad, including two Reuters news staff.
Later that year, the group released over 90,000 secret documents detailing the U.S.-led military campaign in Afghanistan, followed by almost 400,000 internal U.S. military reports detailing operations in Iraq.
Those disclosures were followed by the release of more than 250,000 classified cables from U.S. embassies. It would go on to add almost three million more diplomatic cables dating back to 1973.
Per Samuelson, one of Assange's Swedish lawyers, said if the U.N. panel judged Assange's time in the embassy to be custody, he should be released immediately, even though the panel's decision is not legally binding.
"It is a very important body that would be then saying that Sweden's actions are inconsistent with the European Convention on Human Rights. And it is international common practice to follow those decisions," Samuelson told Reuters.
The Swedish government and prosecutor declined to comment.
Lennart Jansson, the charge d'affaires at the Swedish embassy in Canberra, declined to comment on the announcement.
Since Assange's confinement, WikiLeaks has continued to publish documents on topics such as the Trans-Pacific Partnership, one of the world's biggest multinational trade deals, which was signed by 12 member nations on Thursday in New Zealand.