A spokesman for Staffordshire Police said the force was aware of the incident at the privately-run jail and was offering support and assistance to G4S.
More top news
The reason for the shooting is unclear, but local media have reported it may be a settling of scores between criminals.
Most parts of the country will experience near unbroken sunshine after a chilly start to the day.
As the investigation into Khalid Masood continues, former acquaintances tell of violent incidents in the murderer's past.