Violence is "spiralling out of control" in UK prisons and incidents are being under-reported, according to a European watchdog.
The report, by the European Committee for the Prevention of Torture, warned there were "weaknesses" in the way violent incidents are recorded.
A wave of serious disturbances and decline in safety standards led to ministers launching a reform programme.
The Ministry of Justice said it was working on boosting prison safety.
The committee, part of the Council of Europe, visited a number of prison establishments - including HMP Doncaster and HMP Pentonville - last spring.
But they found that "the cumulative effect of certain systemic failings was that none of the establishments visited could be considered safe for prisoners or staff".
It said it was "deeply concerned by the amount of severe generalised violence evident in each of the prisons visited", adding that this was both violence between prisoners and attacks on staff by inmates.
'Inhuman and degrading'
At both Doncaster and Pentonville, the delegation "gained the impression that the actual number of violent incidents appreciably exceeded the number recorded".
The report said: "This issue appeared to be particularly acute at Doncaster Prison, where the delegation established that some violent incidents had either not been recorded or recorded as being less serious than they were in practice.
"Moreover, the delegation observed first-hand that violent incidents were not always reported by staff.
"While the number of recorded violent incidents at all prisons visited was alarmingly high, the CPT believes that these figures under-record the actual number of incidents and consequently fail to afford a true picture of the severity of the situation."
In the report, the committee raised concerns about chronic overcrowding and also said there were "inadequate" regimes in every place they visited, with a considerable number of prisoners spending up to 22 hours of each day locked in their cell.
Young people held at one youth offenders institution the committee visited were locked up in cells alone for up to 23-and-a-half hours a day - something they described as "inhuman and degrading treatment".
They said operational safety had been compromised at both HMP Doncaster and HMP Pentonville, partly because of low staff levels or the inadequate deployment of staff.
The committee warned that unless "determined action" is taken to significantly reduce the current prison population, the "regime improvements" envisaged by the authorities' reform agenda would remain unattainable.
They concluded: "The CPT recommends that concrete measures be taken to bring prisons back under the effective control of staff, reversing the recent trends of escalating violence and that a far greater investment in preventing violence be undertaken.
"In particular, this requires a swift reinforcement of staffing levels to provide for a safe environment for prisoners and staff."
Assaults in jails in England and Wales reached a record high of 25,049 in the year to September - equivalent to more than 60 every day, the most recent official statistics show.
A Ministry of Justice spokeswoman said: "We have always been clear that we need to make our prisons safe so offenders can turn their lives around and live as law-abiding citizens.
"That is why we have taken immediate action to stabilise the estate by tackling the issues that undermine security, and are investing £100m annually to boost the frontline by 2,500 officers.
"These measures, along with the work to overhaul our prisons so they become places of reform, will help reduce crime, create fewer victims and safer communities."
The inspection took place before the justice secretary had announced measures to help deal with prison violence.
John Wadham, chairman of the UK National Preventive Mechanism, a collection of bodies which have powers to inspect or monitor places of detention, said of the report: "A great deal of what they say echoes our own concerns.
"Our research shows that more than 124,000 people are detained in the UK on any given day. This report is an important reminder that there is much to be done to improve the conditions in which people are held and how they are treated."