A second successive day of trouble has erupted at HMP The Mount in Hertfordshire.
BBC home affairs correspondent Danny Shaw said armed prisoners had taken over Nash Wing and smashed windows, according to well-placed sources.
Riot-trained teams are being despatched to the jail in Bovingdon village.
The Ministry of Justice (MOJ) said the incident was ongoing. A separate disturbance has also been dealt with at Erlestoke prison in Wiltshire, it said.
On Monday at least 50 cells are thought to have been damaged when violence broke out in two wings at The Mount. No-one was injured on that occasion.
The Mount, near Hemel Hempstead, opened in 1987 and is classed as a category C male prison.
HMP Erlestoke is also a category C men’s prison.