A 14-hour riot involving at least 500 prisoners at the privately run Birmingham prison should have been prevented from escalating within 30 minutes, according to the findings of an official investigation the government previously attempted to suppress.

The mass disturbance broke out on four wings of HMP Birmingham, which was run by the contractor G4S, on the morning of 16 December 2016 after two men climbed on the netting in one of the wings and, along with four other inmates, were able to seize a set of keys.

Chronic staff shortages contributed in part to a breakdown in authority and increasing instability that led to the riot, an investigation led by a former prison service director found. The investigation was commissioned by the prison service.

There were several windows of opportunity in the early stages of the incident when order could have been re-established and the subsequent escalation of violence, which involved serious damage to the four wings, prevented.

The Ministry of Justice announced on Monday it was bringing HMP Birmingham back under government control from G4S. Photograph: Joe Giddens/PA

The report into the investigation was released by the Ministry of Justice on Monday under freedom of information laws on the same day the department announced it was bringing HMP Birmingham back under government control from G4S. Ministers previously refused to publish even a redacted version of the report, citing security concerns.

In a damning summary, the report said: “We formed the view that staff had, over the preceding year and … the preceding few months, become worn down by the chronic staffing shortages at HMP Birmingham, caused by a combination of high levels of sickness, attrition and disorganised deployment.

“Over this time period, they had gradually relinquished authority to the prisoners who were in effect policing themselves much of the time.”

Quick Guide British jails in jeopardy Show Overcrowding The current male and female prison population is 84,255, having almost doubled in 25 years. Analysis suggests jails in England and Wales are holding more than 10,000 prisoners than they were built for, with two-thirds of prisons classified as overcrowded. Self-harm Currently at record levels, with 42,837 incidents – an average of 117 a day – documented during the year to September, an increase of 12%. Already it looks like it will be higher this year with 11,904 incidents recorded during the last quarter, equal to 130 a day. Eight inmates a day will be admitted to hospital. There is a suicide in prison every five days. Assaults Again, at record levels with 28,165 incidents documented during the 12 months to September 2017, a 12% increase. Of these, 7,828 were assaults on staff . Already at unprecedented levels, the rate continues to escalate. During the last quarter of 2017, another record high was set with assaults rising to 86 a day. Staffing numbers The Ministry of Justice is now increasing prison officer numbers following dramatic cuts. Latest figures reveal a rise from 17,955 in October 2016 to 19,925 in December 2017. In 2010 there were 27,650. Reoffending rates The number of adults reconvicted within a year of release stands at 44%. For those serving sentences of less than 12 months, this increases to 59%.

A national prison service unit was not informed of the disturbance promptly, leading to delays in dispatching specially trained Tornado Squad teams to the prison.

The report said prisoners did not plan the riot and that it was the product of months of building dissatisfaction among inmates over access to healthcare, as well as general drug use and violence on the wings.

However, it also found it was “the absence of leadership and prompt action by staff present that allowed the incident to initially develop”.