The Australian will be immediately arrested by officers from the Metropolitan Police under an agreement secretly struck between officials from the South American country and the UK, the post claimed late on Thursday night.
Mr Assange has been living at the embassy for seven years since seeking refuge there to avoid extradition to Sweden, where he faced sexual assault allegations.
Although those accusations have since been dropped, the 47-year-old has remained living in the Knightsbridge building for fears of being extradited to the US, where he faces charges over the release of sensitive government files.
The new WikiLeaks tweet said: “A high level source within the Ecuadorian state has told @WikiLeaks that Julian Assange will be expelled within ‘hours to days’ using the #INAPapers offshore scandal as a pretext--and that it already has an agreement with the UK for his arrest.”
The so-called INA Papers are a collection of documents, leaked to an Ecuadorian politician, which have implicated the country’s president Lenin Moreno in a corruption and tax-avoidance scandal.
WikiLeaks and Mr Assange – who occupies a third of the entire embassy – say they have nothing to do with the leak.
Ecuadorian officials denied a decision had been taken to expel Mr Assange. The foreign ministry said in a statement that it "doesn't comment on rumours, theories or conjectures that don't have any documented backing".
However, rumours officials may be losing patience with their permanent house guest have been mounting for some time.
They have repeatedly complained that he has caused damage by playing football and skateboarding in the building and in March last year cut his internet access amid concerns he was interfering in the country’s foreign policy.
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