Julian Assange’s father has claimed his son was “harassed” by a prison cell search the day before the start of his court battle against extradition.

John Shipton visited the WikiLeaks founder at Belmarsh Prison in south-east London for two hours on Sunday.

Speaking to reporters afterwards outside, Mr Shipton demanded Assange be released on bail.

John Shipton after visiting son Julian Assange at HMP Belmarsh in London (Yui Mok/PA)

“For the life of me I can’t understand why Julian Assange is in jail having committed no crime, with family here that he can come and live with,” he said.

“Bail ought to be given immediately if the extradition order isn’t dropped.

“Julian had a harassment today. He goes to court tomorrow. They searched his cell this afternoon just before he came down to see us.

“This plague of malice that emanates from the Crown Prosecution Service to Julian Assange must stop immediately.”

Mr Shipton was accompanied on his visit by Greek economist Yanis Varoufakis, with both met by representatives from Reporters Without Borders, an organisation that defends freedom of information, as they left the prison.

Greek economist Yanis Varoufakis speaks to the media after visiting Julian Assange (Yui Mok/PA)

Mr Shipton said it was “distressing” and “very upsetting” to see his son in jail, adding that he had told him his whole family would be supporting him.

Mr Varoufakis said Assange was in a “very dark place” due to spending more than 20 hours a day in solitary confinement.

Describing the Australian as a “force of nature”, he said he was not being allowed to exercise in the gym with other inmates.

“We have to stop this extradition in the interests of 300 years of modernity, 300 years of trying to establish human rights and civil liberties in the west and around the world,” Mr Varoufakis added.

Christophe Deloire, secretary general of Reporters Without Borders, said: “The question is not are you part of the family or not, do you like Julian Assange or not. The question is that tomorrow in this courtroom, journalism will be in danger.

“Because if Assange would be extradited to the US, it would be the sign that journalism is considered as espionage and it would endanger all journalists who want to cover the lies of governments whatever the country.”

Assange, 48, is wanted in the US on 18 charges over the publication of US cables a decade ago and if found guilty could face a 175-year prison sentence.

He is accused of working with former US army intelligence analyst Chelsea Manning to leak hundreds of thousands of classified documents.

He is being held in Belmarsh as he awaits the start of a full extradition hearing at the nearby Woolwich Crown Court on Monday.

On Sunday evening, some Assange supporters were setting up tents by the side of the road outside the court building.

Julian Assange’s father has called for him to be released on bail

Last week Mr Shipton warned his son’s extradition would be akin to a “death sentence”.

He addressed the hundreds of Assange supporters after they marched through central London to Parliament Square on Saturday to protest against his son’s potential extradition.

Musician Roger Waters, Mr Varoufakis and fashion designer Vivienne Westwood also gave speeches in support of Assange.

More than 40 international legal experts have written to Prime Minister Boris Johnson demanding the “rule of law be upheld” before Assange’s hearing on Monday, claiming he has not had proper access to his legal team.

The letter was handed in to 10 Downing Street on Saturday and also urged the British legal community to act “urgently” to secure Assange’s release.