The public is being put at risk by a "political and policy failure" in prisons which risks boiling over this summer, a watchdog warned as the Government admitted more prisoners would have to share cells to cope with an unexpected rise in numbers.
Chief inspector of prisons Nick Hardwick issued a stark warning that cuts had left the system so stretched that more inmates were killing themselves or getting deliberately sent to punishment blocks to escape crowded conditions.
He told BBC Radio 4's Today he was "very concerned" about the impact of a hot summer and said ministers must find significantly more resources or cut the number being put behind bars.
Justice Secretary Chris Grayling rejected the criticisms, insisting the number of assaults and cases of self-harm were falling and that 2,000 extra prison places were being built.
He conceded that the Government had been taken by surprise by a recent surge in demand for cell space, blaming a spate of historic sex abuse cases.
But he defended his decision to order dozens of already full and overcrowded prisons to take 440 extra offenders between them as a "sensible precaution" to deal with the unanticipated squeeze.
Labour has accused the Government of incompetence, blaming the shortages on the decision to close several prisons before replacement capacity was available.
In damning assessment, Mr Hardwick said: "Because of staff shortages, men are spending 23 hours a day, two or three to a cell with a shared toilet, locked up in this heat and that is causing huge tensions.
"We have got to cut our coat according to our cloth. You either have to put in significantly more resources to cope with the numbers we've got or you have to reduce the population.
"We are looking at a long hot summer. The situation is already extremely serious and adding pressure to it causes me great concern."
Pointing the finger directly at Westminster, he said: "We need to be clear about this: this is a political and policy failure.
"This isn't the fault of the operational staff on the ground or managers, this is because the demands on the system have now completely outstripped the resources available to meet them.
"There is hardly any activity going on, resettlement has ground to a halt. Prisoners, staff and the public are at risk because of it."
He said he was less concerned about the capacity of jails than the resources needed to ensure safety and prevent reoffending.
"On both of those accounts, at the moment, the system is failing," he said.
Mr Grayling told the programme: "I'm afraid I just simply disagree with a lot of what Nick Hardwick has just said."
Assaults and self-harm were down on two years ago, he said, and there were at present 1,000 spare places.
"We have had an increase in recent weeks. A number of factors - things like, for example, the increased number of convictions for historic sex abuse.
"Things are a little bit tighter than we expected them to be. We might later in the summer have a capacity issue.
"So what I am doing now, in advance, is taking precautions to make sure that if that happens we can cope.
"This is in marked contrast to 10 years ago. When the last government went through this they had to let prisoners out 18 days early to cope with the pressures."
He downplayed the impact of ordering 40 prisons - 34 of which are already at or over capacity - to take more inmates.
"We will have a few hundred prisoners more who have to share a cell for a few weeks. That's what overcrowding actually means," he said.
"The amount of work available in prisons is expanding significantly, we have put in efforts to increase the availability of education," he added, and supervision for those released from short sentences would also begin this year
Defending the cuts programme, he said he was following the recommendations of prison governors.
"The changes we have put in place in the last few months, although difficult and challenging in order to meet budget restraints, are those that were recommended by our own governors as the way forward.
"I have done what was recommended to us by our teams. I know it's difficult, I know it's challenging for many prison officers."
Shadow justice secretary Sadiq Khan said: "When the chief inspector of prisons warns of the threat of serious disturbances we should all sit up and listen, including Chris Grayling.
"This prisons overcrowding chaos is solely of the Government's own making. Despite our warnings, they've closed down 17 prisons, taking 5,000 spaces out of use.
"Violence is up, deaths in custody are at record highs and staff numbers have been slashed. The idea of prisons being able to rehabilitate offenders so they stop reoffending in these circumstances is a joke."