An Ecuadoran prosecutor will quiz the founder of the secret-spilling website at the red-brick building where he has been holed up for more than four years, with Swedish prosecutor Ingrid Isgren and a Swedish police inspector also attending, officials said.
The 45-year-old Australian sought refuge in the central London embassy in June 2012 after Swedish prosecutors issued a European arrest warrant against him, over allegations of rape and sexual assault filed by two women who met Assange during a 2010 trip to Sweden.
He denied the claims, saying they were politically motivated, and insisting his sexual encounters with the two women were consensual.
He has refused to travel to Sweden for questioning, fearing he would be extradited to the United States over WikiLeaks' release of 500,000 secret military files on the wars in Afghanistan and Iraq.
Swedish prosecutors dropped the sexual assault probe last year after the five-year statute of limitations expired.
But they still want to question him about the 2010 rape allegation, which carries a 10-year statute of limitations.
A Swedish official source said the questioning was expected to begin at around 1000 GMT. The investigators intend to take a DNA sample, subject to his agreement.
"It's planned to last a few days," Assange's lawyer Per Samuelsson told AFP, adding that it was too early to say what might arise from the meeting or what would be made public.
It will be the first time he has been interviewed over the matter since initial questioning by Swedish police at the time of the allegation.
Assange, speaking through his lawyer, has said he welcomes the "chance to clear his name" and hopes the investigation will subsequently close.
In May, a Swedish court reaffirmed the arrest order, rejecting the finding of a UN working group that his confinement in the Ecuadoran embassy amounted to arbitrary detention.
In the days since the US election, supporters have launched a petition calling on president-elect Donald Trump to pardon Assange by "absolving him of any crimes alleged against him" - an apparent reference to the military leaks.
The petition on the change.org website, which has gathered more than 16,500 signatures, hails Assange as a "hero" for exposing the "corruption of those who presume to rule us".
In pictures: Julian Assange's 'arbitrary detention'
In pictures: Julian Assange's 'arbitrary detention'
1/14 In pictures: Assange's 'arbitrary detention'
Wikileaks founder Julian Assange speaks from the Ecuadorian Embassy on December 20, 2012 in London, England.
2/14 In pictures: Assange's 'arbitrary detention'
Supporters of Julian Assange show banners as they wait for his appearance opposite the Ecuadorian Embassy yesterday (Frank Augstein/AP)
3/14 In pictures: Assange's 'arbitrary detention'
A supporter of WikiLeaks founder Julian Assange demonstrates outside Ecuador's embassy in central London on February 5, 2016
4/14 In pictures: Assange's 'arbitrary detention'
Assange speaking to the media outside the Ecuador embassy in 2012
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Fashion designer and political activist Vivienne Westwood to visit Wikileaks founder Julian Assange
6/14 In pictures: Assange's 'arbitrary detention'
Members of the media wait outside the Ecuadorian embassy where Wikileaks founder Julian Assange continues to seek asylum in February 2016.
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Supporters of Assange wait for the arriveal of Ecuadorian Foreign minister Ricardo Patino (Andrew Cowie/AFP/Getty Images)
8/14 In pictures: Assange's 'arbitrary detention'
A supporter outside the embassy, which is guarded by police. The 24-hour operation is said to have cost the British taxpayer £3 million (Tal Cohen/EPA)
9/14 In pictures: Assange's 'arbitrary detention'
Supporters of the WikiLeaks founder Julian Assange at a protest outside the Ecuadorean embassy yesterday, where he has been holed up for three years
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Supporters have appeared at the embassy numerous times over the last year, especially when Mr Assange is set to appear, as he did yesterday (Frank Augstein/AP)
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The Foreign & Commonwealth Office have made clear that: “The UK has a legal obligation to extradite Mr Assange to Sweden” (Chris Helgren/Reuters)
12/14 In pictures: Assange's 'arbitrary detention'
Demonstrators hold banners outside the Ecuadorean Embassy in London, where Wikileaks founder Julian Assange is staying
13/14 In pictures: Assange's 'arbitrary detention'
14/14 In pictures: Assange's 'arbitrary detention'
Meanwhile Assange's lawyer said he had made "repeated requests" for an interview with police to address the rape claim, though Ecuadoran prosecutors say a hearing scheduled for October was postponed at the Australian's request.
"Julian Assange has always wanted to tell his version to the Swedish police. He wants a chance to clear his name," Samuelsson told AFP.
The legal grilling comes after WikiLeaks returned to the spotlight with the leak of tens of thousands of emails from the US Democratic Party and presidential candidate Hillary Clinton's campaign in the final weeks of the race for the White House.
Assange defended the publication, denying links with Russia and claims that his website was trying to influence the US vote which saw Republican Trump elected.
Tensions with his Ecuadoran hosts have been growing, with the leaks prompting the embassy to cut Assange's internet access, citing respect for "non-intervention" in the affairs of other states and their electoral processes.
WikiLeaks released medical records in September claiming Assange's mental health was at risk if he remained any longer in the embassy.