The prison in southeast London became one of the country's best-known jails after Great Train Robber Ronnie Biggs and career criminal Charles Bronson -- dubbed "the most violent prisoner in Britain" -- were held there.
Opened in 1991 next to Woolwich Crown Court, it has frequently been used in high-profile national security cases.
Radical cleric Anjem Choudary, who was convicted in 2016 of encouraging support for the Islamic State group, spent some of his sentence there.
Belmarsh earned the moniker "the UK's Guantanamo Bay" in the period following the 9/11 attacks after it was used to detain a number of people under anti-terror laws.
The prison, which has a capacity of around 900, houses inmates on remand and serving short sentences as well as those convicted of the most serious offences.
It can hold prisoners "requiring specific management arrangements because of their public and media profile," according to an inspection report last year.
The report said the facility suffered from a "significant shortage of frontline staff" which had resulted in a "severely depleted daily regime" that limited the time inmates could be given outside their cells.
Belmarsh may be an upgrade for Assange on his previous British prison digs.
Deemed "the most overcrowded prison" in England at its last inspection in 2018, the 19th-century facility holds around 1,600 inmates, with most prisoners sharing a cell designed for one person.
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