Detailed plans to kidnap a prison officer in a bid to escape from the Isle of Wight Prison, were found in the cell of a Muslim inmate.
An internal Prison Service document, seen by the BBC, reveals an Islamic State, or ISIS, flag was also found in the cell at the jail's Parkhurst site.
The prisoner was placed in the jail's close supervision unit.
A Ministry of Justice (MoJ) spokesman said security measures had been shown to be "robust" by the discovery.
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.
In the bulletin for the week ending 19 October, it says "acting on intelligence a prisoner's cell was searched".
"Detailed escape plans involving taking a member of staff hostage were discovered."
It says activity at the prison was restricted "whilst the accommodation and his associates were searched".
The MoJ said "a very basic description" of the interior of the prison was found in the inmate's cell by "vigilant" staff, demonstrating the "effectiveness of its security measures".
HMP Isle of Wight was created in 2009 with the merger of Parkhurst, Albany and Camp Hill prisons.