The 40-year-old Australian, whose whistleblowing website has enraged governments around the world, has been fighting a bitter legal battle ever since he was arrested in London last December on a European arrest warrant.
The High Court will announce at 9.45 GMT whether it will uphold a ruling in February by a lower court that Assange should be sent to face questioning by Swedish authorities over claims of sexual assault against two women.
The enigmatic WikiLeaks boss has been living under strict bail conditions at the mansion of a supporter in eastern England, including having to wear an electronic ankle tag and observe a strict dusk-to-dawn curfew.
The long-awaited decision, which has been deferred since the High Court heard Assange's appeal in July, is not necessarily the final chapter in the saga, but it will be difficult for either side to lodge an appeal.
Depending on the verdict, either Assange or the Swedish authorities can theoretically take the case a step further to the Supreme Court in London, the highest legal authority in the land.
"If you fulfill certain criteria you may get permission to appeal to the Supreme Court," a spokeswoman for the office of the Judiciary of England and Wales told AFP.
But leave to appeal can only be granted by either the High Court or the Supreme Court, and then only if it there is a point of law of general public importance.
Assange denies the allegations and claims they are politically motivated.
His legal team, led by top human rights lawyer Gareth Peirce, was not available for comment ahead of the ruling.
Assange shot to fame last year when WikiLeaks released tens of thousands of classified diplomatic files allegedly obtained by a US serviceman who is now in prison in the United States.
The move infuriated Washington as many of the files related to the wars in Iraq and Afghanistan, while others contain frank and sometimes embarrassing assessments of world leaders made by US diplomats.
But Assange also achieved a different kind of notoriety when the allegations of sexual assault were made against him by two Swedish women in August 2010.
In the last hearing in July, lawyers representing Swedish prosecutors rejected claims that a rape allegation against Assange, made by one of the women, would not be valid under English law.
Swedish lawyers accused Assange's legal team of "winding English law back to the 19th century" with their definition of consent.
Assange's lawyers have also claimed that the European warrant under which he was arrested was invalid because he is only wanted for questioning and has not been charged by Swedish authorities.
In an autobiography published in September despite his efforts to halt the project, Assange repeated his denial of the rape allegations.
"I did not rape those women and cannot imagine anything that happened between us that would make them think so, except malice after the fact, a joint plan to entrap me, or a terrifying misunderstanding that was stoked up between them," he wrote in the 250-page book.
"I may be a chauvinistic pig of some sort but I am no rapist, and only a distorted version of sexual politics could attempt to turn me into one."
WikiLeaks is meanwhile struggling for survival.
Assange said last month the site had been forced to suspend publishing classified files after its funding was blocked could have no option but to shut down by the end of the year.
It has seen its funding plunge by 95 percent due to an "arbitrary and unlawful financial blockade" mounted by companies including Visa and MasterCard since December last year, Assange said.