An outbreak of disorder at the country's largest prison went on for nearly twice as long as previously thought.
Trouble broke out on a wing at G4S-run HMP Oakwood, near Wolverhampton, at 5pm yesterday evening and was not resolved for nine hours until 2am, the Ministry of Justice and G4S said. The private operator previously said the incident was over in five hours.
The incident, which took place in one wing of the jail, involved up to 20 inmates, who threatened officers and damaged cells and prison property. No prison staff were injured, although one prisoner has been treated for minor injuries.
HMP Oakwood, which houses more than 1,600 category C prisoners, was the scene of a number of rooftop protests last year and was slammed by inspectors during a surprise visit.
G4S and the MoJ denied reports that prison staff had been taken hostage, labelling such claims as "completely untrue".
A joint statement read: " The safety of our personnel and those prisoners in our care is our top priority, and we are grateful to our colleagues who were able to help us bring the incident to a close safely, and effectively.
"Established incident procedures were followed correctly and worked as they were meant to.
"As an investigation has now commenced into the reasons for this disruption, as well as a criminal investigation, it would be inappropriate to comment further at this time."
Standard procedures were initiated to deal with the incident after the MoJ was informed, with rapid response teams deployed. Staffordshire Police were also informed.
A spokesman for Staffordshire Police said the force was offering support and assistance to G4S.
The prison - the largest in England and Wales - opened in April 2012 as a training prison next to the existing HMP Featherstone and HMP Brinsford near Wolverhampton.
In a report published in October, HM Inspectorate of Prisons (HMIP) reported inexperienced staff and high levels of violence and self-harm at the jail - dubbed ''Jokewood'' by prisoners.
Chief Inspector of Prisons Nick Hardwick warned there were ''real risks if matters were allowed to drift'' at the prison.
At the time G4S said improvements were being made but admitted launching the prison was a "complex and challenging operation".
G4S - well-known for its botched handling of its Olympics security contract - has been under review by the Government following revelations it overcharged for criminal-tagging contracts.
The government has since announced that electronic monitoring will handed to another firm on an interim basis at the end of the financial year.