The Australian founder of the whistleblowing website says comments Gillard made in 2010 that the leaking of US diplomatic cables was "illegal" and "grossly irresponsible" had hurt WikiLeaks' financial viability.
"We are considering suing for defamation so I have hired lawyers in Sydney and they are investigating the different ways in which we can sue Gillard over this statement," he told Australian activist group GetUp in an interview.
Assange has been holed up in the Ecuadorian embassy in London since claiming asylum on June 19 in a bid to avoid extradition to Sweden, where he faces questioning over alleged sex crimes.
He said the organisation had not been convicted of breaking any laws yet Gillard's comments had been used against WikiLeaks.
"MasterCard Australia, in justifying why it has made a blockade preventing any Australian MasterCard holder from donating to WikiLeaks, used that statement by Julia Gillard as justification," he said.
"So the effects of the statement are ongoing and they directly affect the financial viability of WikiLeaks."
Assange, who plans to run for the Australian Senate in elections due next year, denies the sex allegations and fears Sweden would extradite him to the United States, where his supporters say he could receive the death penalty.
WikiLeaks deeply embarrassed the US government in 2010 by publishing huge caches of confidential documents on the wars in Iraq and Afghanistan and more than 250,000 diplomatic cables from US embassies around the world.
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Ecuador granted him asylum on August 16 but Britain has denied him safe passage out of the country.
Assange again accused the Australian government of offering him little help -- an allegation it rejects.
"I haven't seen any member of the Australian embassy or consulate since I was in prison in 2010. And even then, all they did was bring some notepads, pens, etc," he said.