A roof in Wandsworth Prison was set on fire after prisoners threw burning paper on to it.
Four fire engines were called to the prison in Heathfield Road, Wandsworth, at 1.20am today.
The fire was caused by prisoners throwing burning paper out of the windows, which set a small roof alight.
It is believed there was a disturbance in the prison which provoked the incident.
No oner was injured, with fire crews leaving the scene at 4.30am.
It comes in the same week as HMP Wandsworth was declared the most overcrowded prison in the country according to a report by the Howard League for Penal Reform.
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