An investigation has been launched after two prisoners were filmed being punched in the face.
It comes after a national newspaper published the footage which was said to have been filmed at HMP Elmley in Church Road, Eastchurch.
One of the men is thought to have suffered a broken jaw in the incident, which happened on May 3.
A man is seen spitting blood in a sink after the attack. Picture: Sunday Mirror
The video shows the two men saying they are going to be hit in the face in exchange for cannabis before they are struck.
At one point a man is seen falling on the floor, before getting up and spitting what looks like blood into a sink.
His attackers are heard laughing before asking him if he's OK.
A punch is seen thrown at one of the men. Picture: Sunday Mirror
One of the men was thrown to the floor with the force of a punch. Picture: Sunday Mirror
Police are treating the incident as GBH and say enquiries are ongoing after a report from the category C prison.
A Ministry of Justice spokesman said: "We do not tolerate violence of any kind and take swift and robust action against perpetrators."
An Independent Monitoring Board report, published in April, highlighted the impact of cuts to staffing levels at Elmley.