Julian Assange is being held in one of Britain's most high profile prisons, in what his WikiLeaks colleague describe as "appalling" conditions.

The 47-year-old is in "general" solitary confinement because he mostly spends 23 hours a day in his cell, Kristinn Hrafnsson says.

Julian Assange last month (AAP)

Belmarsh Prison in Southeast London. (AAP)

The WikiLeaks editor-in-chief says the situation is "unacceptable".

"We are worried about Julian Assange. We are hearing that the situation in Belmarsh prison, in Her Majesty's prison, is appalling because of austerity and cutbacks," he said outside Westminster Magistrates' Court on Wednesday.

"For the last weeks since he was arrested he has spent 23 out of 24 hours a day in his cell most of the time.

Michael Adebolajo one of the two men who murdered British soldier Lee Rigby in a broad daylight attack. (AAP)

"That is what we call in general terms solitary confinement. That's unacceptable. That applies to most of the prisoners in that appalling facility. It is unacceptable that a publisher is spending time in that prison."

Among inmates known to have been housed in the prison are Michael Adebolajo, one of the killers of soldier Lee Rigby, Finsbury Park terror attacker Darren Osborne, and radical preacher Anjem Choudary before his release from jail last year.

Belmarsh, which sits next to Woolwich Crown Court in southeast London, is one of only three high-security "core" local prisons in England and Wales.

Other notorious previous inmates include the Great Train Robber, Ronnie Biggs and armed robber Charles Bronson.

It opened in 1991 and had an operational capacity of 910 prisoners as of 2008, according to the Ministry of Justice.

An inspection report in 2018 showed that levels of violence had increased at Belmarsh since it was last assessed three years previously.