A prisoner described by British police as an "IRA sympathiser" is being questioned following the explosion of a small incendiary device on Saturday night in his cell at Parkhurst Prison on the Isle of Wight.
Senior sources at the prison, from where three prisoners escaped in January 1995, said they believed the device was made with matches and had been placed behind a viewing flap in the cell door in order to injure anyone who opened it.
Other prison sources believe the device may have been crafted with the help of crude instructions from a chemistry book in the prison library.
The 27 year old prisoner, who has not been named, was in the health care unit of the prison at the time of the explosion.
A prison spokesman described the device as exploding "with a loud bang and a bright flash".
He said: "Two officers entered the cell as the prisoner had become rowdy. The device went off when they, were inside but there were no injuries."
The director general of the Prison Service, Mr Richard Tilt, praised his staff for their quick response to the incident, which he described as "unpleasant". He added: "The situation was over quickly, due to the work of my staff. The device caused very little damage."