By midday, two buildings were still ablaze, though fires had been extinguished at several other buildings, which were heavily damaged.
The prison's staff retreated and specialist officers were called in to handle the riot, the Ministry of Justice said. There were no immediate reports of injuries to staff or inmates.
The prison houses up to 500 offenders with less than two years remaining on their sentences to prepare them for release.
Mark Freeman, deputy general secretary of the Prison Officers Association, said the riot started after some prisoners refused to take breath tests. At the time, only two prison officers and four support staff were on duty, he said.
Freeman said a large amount of alcohol had been found at the prison.
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