Frances Crook, from the Howard League for Penal Reform, said the report's findings were "dreadful" and added: "Isis is only six years old, but it is violent, filthy, overcrowded, understaffed, and failing to help people to lead crime-free lives.
“The picture painted by inspectors is nothing short of a disgrace."
Michael Spurr, the chief executive of the National Offender Management Service, said that "progress" had been made since the arrival of a new governor.
He added: "More prisoners are now accessing education, training or work facilities, providing offenders with the opportunities they need to turn their backs on crime."