A prisoner was found with a home-made Islamic State (IS) flag and plans to kidnap a prison officer while escaping, it has been reported.
A description of the inside of the Parkhurst site of Isle of Wight Prison was found in the cell of a Muslim inmate, leading to reports that a breakout was being planned.
The prisoner has now been segregated.
The BBC reported that detailed plans to kidnap a prison officer in a bid to escape were found in the prisoner's cell.
The broadcaster said it had seen an internal Prison Service document revealing that a black flag of IS, also known as ISIL or ISIS, was also discovered.
Details of the incident were contained in the weekly security bulletin of the National Offender Management Service, which is compiled for all prisons in England and Wales, the report said.
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In the bulletin for the week ending October 19, the document reportedly says "acting on intelligence a prisoner's cell was searched".
"Detailed escape plans involving taking a member of staff hostage were discovered."
The bulletin is said to report that activity at the prison was restricted "whilst the accommodation and his associates were searched".
A Ministry of Justice spokesman said today: "Vigilant staff found a very basic description of the inside of the prison during a cell search at HMP Isle of Wight.
"This clearly demonstrates the effectiveness of our robust security measures. The prisoner is now in segregation."
A source said there was no threat to prisoners or staff and no other items apart from the description of the inside of the prison were found during the search.