Assange has been ensconced in Ecuador's embassy in London since 2012 to avoid extradition to Sweden.
Chief prosecutor Ingrid Isgren had been scheduled to visit the embassy on Wednesday and Thursday this week to question the WikiLeaks founder over rape allegations in the Nordic country.
But the 43-year-old's Swedish lawyers told the TT newswire late on Wednesday that the meeting would not be going ahead as planned.
“I was informed that the interrogations would not be held because they had not received the required permissions from Ecuador,” said Thomas Olsson.
Fredrik Berg, press spokesman at Sweden's Prosecution Authority (Åklagarmyndigheten), did not want to comment, but said there had been problems with a number of documents that had delayed the process.
“We will do everything to ensure the interrogation happens in June or July,” he insisted.
Meanwhile, Ecuador said it was still considering Swedish prosecutors' request.
Ecuador is evaluating the request “in the spirit of judicial cooperation” and will make a decision based on international law and “Ecuadorian jurisdiction in the area of asylum rights,” the foreign ministry said in a statement.
TIMELINE: Julian Assange sex allegations
Swedish prosecutors said on Monday that they had submitted a request to British and Ecuadoran authorities to question Assange in June and July at the embassy, as reported by The Local.
Sweden issued an arrest warrant for Assange in 2010 following allegations from two women in Sweden, one who claimed rape and another who alleged sexual assault.
The 43-year-old has refused to travel to Sweden because he fears the country would send him to the United States, where an investigation is ongoing into WikiLeaks' release in 2010 of 500,000 classified military files on the wars in Afghanistan and Iraq and 250,000 diplomatic cables which embarrassed Washington.
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He has long offered to be interviewed by prosecutors at the Ecuadorian embassy or by video link.
Swedish prosecutors had insisted that he travel to Sweden to answer to the allegations, but in March they finally agreed to question him in London, marking a significant U-turn in the case that had been deadlocked for nearly five years.
Assange's lawyer Per Samuelsson told The Local at the time that the Australian had described prosecutors' decision to question him in London as "a great victory".
Prosecutors said they had changed their stance because some of the alleged offences will reach their statute of limitation in August.