An inmate at HMP Peterborough was put into segregation after plans to build a pipe bomb were recovered in a cell.
Police were called to the jail in Saville Road, Westwood in March following the alarming discovery.
The hand drawn plans and items were found during a routine search of cells on March 16. They were found in a cell in House Block Five at the Sodexo run prison.
A report in The Sun newspaper said ‘the prison governor and police were called because of the fear the finds were ‘terror related.’
Glyn Travis of the Prison Officers Association said: “The find of dangerous articles and items is an essential.
“Prison officers have done their job professionally and possibly prevented an even bigger threat.
“Unfortunately, with fewer staff, these types of searches are harder to do on a regular basis.”
A HMP Peterborough spokesperson said: “Safety and security in our prison is our top priority.
“Staff responded to this incident quickly and appropriately, and reported it to the police.”
The discovery was not assessed to be a threat, and no metal piping - a key part of a pipe bomb - was found by prison staff.
A spokesman for Cambridgeshire police said they could not find a record of the incident on their logs.
HMP Peterborough houses 840 inmates, and is the only prison in the country to have both male and female prisoners. It is a category B prison for male inmates, and a closed prison for female and young prisoners.
In July 2015 the prison was given a glowing report by Government inspectors, who rated safety at the jail at the highest level.