A video purportedly showing a gaunt Julian Assange inside a maximum security British prison has been leaked.

Key points: In the video, Julian Assange is seen speaking to other prisoners inside the prison

In the video, Julian Assange is seen speaking to other prisoners inside the prison It comes after Assange was moved to a health-ward inside the maximum security facility

It comes after Assange was moved to a health-ward inside the maximum security facility It is forbidden to have filming or recording devices inside a prison

The leak comes as the Australian's father paid a visit to his son in prison and Britain confirmed the United States had formally asked them to extradite the Wikileaks founder to face charges he conspired to hack government computers and violated espionage law.

"Mr Assange was arrested in relation to a provisional extradition request from the United States of America," a spokesperson for the Home Office said.

"He is accused of offences including computer misuse and the unauthorised disclosure of national defence information.

"We have now received the full extradition request."

The five-and-a-half-minute-long clip was uploaded to YouTube on Friday by Russian media outlet Russia Today, which claims the video was filmed by an inmate inside Belmarsh prison in south-east London.

During the clip, which the ABC has not been able to independently verify, the WikiLeaks founder is seen talking with other prisoners while wearing a blue sweatshirt and grey jeans.

A timestamp on the video shows the date of July 7, 2017, but RT claims "this discrepancy was due to a faulty setting on the recording device and that the video was indeed shot inside Belmarsh prison".

Assange is serving a 50-week jail sentence at Belmarsh after being found guilty of skipping bail and avoiding police by spending seven years in the Ecuadorian embassy in London.

He missed a court appearance at the end of May due to ill health, prompting WikiLeaks to release a statement that Assange had been moved to a health ward inside the maximum security facility.

"During the seven weeks in Belmarsh his health has continued to deteriorate and he has dramatically lost weight," it added in a statement in May.

"The decision of prison authorities to move him to the health ward speaks for itself."

The UK's Ministry of Justice declined to comment to the ABC about the unverified video, but it is forbidden to have filming or recording devices inside a prison and any prisoner can expect more jail time if caught with one.

WikiLeaks editor-in-chief Kristinn Hrafnsson said it was "quite obvious" the video was taken before Assange was transferred into the health ward.

"We who know him actually recognise that he has lost a lot of weight and it's visible," he said on Tuesday.

Assange is next due to appear in court on June 14.

John Shipton, father of Julian Assange, speaks to the media outside Belmarsh Prison. ( ABC News: Dan Loh )

Family shows support for Assange

On Tuesday afternoon Assange was visited at Belmarsh prison by his father, John Shipton, and Chinese artist and activist Ai Weiwei, who both raised concerns about the amount of weight the 47-year-old had lost while incarcerated.

"He's lost about 10kg of weight and he's psychologically under a lot of stress and pressure, but he's determined and [his] spirit's very strong and he'll be back in the fight very soon," Mr Shipton told the ABC.

He said Assange's health condition had "stabilised" and he believed he would move out of the health facility in the next few days.

Despite his improving health, Mr Shipton said it was "extremely difficult" for his son to prepare for his defence against extradition to the US on espionage charges while inside the jail.

Julian Assange's supporters protest outside court last month. ( ABC News: Lincoln Rothall )

"There are some problems in being here, in that it's impossible to prepare a case because you only get two social visits a month, no access to a library, no access to computers," Mr Shipton said.

"So it's extremely difficult to prepare a case in those circumstances in Belmarsh."

Mr Shipton appeared upset outside the prison after meeting his son, and wiped away tears.

"Julian is a gentle intellectual in a maximum security prison," he said.

"He was an asylee, and there are rules and conventions governing asylees, dragooned out of the Ecuadorian embassy, sold by the Ecuadorians, an Australian citizen sold by the Ecuadorians for an IMF loan."

He called on the Australian Government to "diplomatically defend an Australian citizen", and also revealed Assange had asked him to move from his home in Melbourne to the UK.

"He just said could I move over to the UK and I said 'yes, I'll be here in August'," Mr Shipton said.

"I'm delighted."

ABC/Reuters