Appearing by video leak to give evidence on Mr Assange’s behalf at an extradition hearing at the Old Bailey in London, Eric Lewis said if the 49-year-old was found guilty of the charges he faces he could be sentenced to up to 175 years. 

“We are looking at a sentence somewhere between 20 years, if everything goes brilliantly, to 175 years which the government could easily ask for,” he said on Monday.

Mr Lewis, a senior partner at DC law firm Lewis Baach Kaufmann Middlemiss in Washington DC, added: “All signs point to a very long sentence, measured in many decades.” 

Mr Assange has been charged with 18 counts in the US, most gravely of plotting to hack computers and conspiring to obtain and disclose national defence information. 

It is alleged he plotted with then army intelligence analyst Chelsea Manning to crack a scrambled password to a classified US Department of Defence computer. Mr Assange’s defence has claimed that his prosecution is politically motivated and his supporters say he should be given the same protection as a journalist. 

Among the most notable of the information he and his organisation made public was video footage filmed in Iraq in 2007 that showed US helicopters attacking and killing a group of unarmed civilians, among them two Reuters employees, Namir Noor-Eldeen and Saeed Chmagh. A dozen people were killed.

Ms Manning served seven years in jail between 2010–2017 after being charged with more than a dozen crimes, after leaking 750,000 classified, or unclassified but sensitive documents to WikiLeaks. She was originally sentenced to 35 years, but that was commuted by Barack Obama shortly before  he left office. 

Mr Assange was arrested on April 11 last year at the Ecuadorean Embassy in London, after US prosecutors in the Eastern District of Virginia unsealed a criminal case against him, alleging he conspired with Ms Manning to commit computer intrusion. 

He was sentenced to 50 weeks in prison for breaching his bail conditions when he took refuge in the Ecuadorian Embassy in 2012 to avoid extradition to Sweden over sexual assault allegations, allegations he denied. Mr Lewis claimed it had been assumed the Department of Justice would not prosecute Mr Assange, but that changed under the Trump administration.

 Mr Lewis suggested attorney general William Barr was “directing from the top down from his office to the Eastern District of Virginia to be much tougher on leakers, including Mr Assange”. 

He said the later addition of multiple espionage charges was “an abuse of the federal law enforcement authority”. It meant if convicted, Mr Assange would face an effective life sentence. Judge Vanessa Baraitser adjourned the case until Tuesday. 

Additional reporting by agencies