The chief inspector of prisons has attacked the state of prisons in England and Wales after dozens of full jails were told to take more inmates.
Nick Hardwick said "political and policy failure" was behind dangerous overcrowding in the publicly-run jails.
All but six of 40 jails told to help house 440 extra inmates are full or overcrowded. The justice secretary called the move a "precaution".
Some 600 extra places are also being purchased in private prisons.
The 40 publicly-run jails told to raise their "operational capacity" include Bedford, Durham, Leeds, Leicester, Lincoln and three prisons in London - Brixton, Pentonville and Wandsworth - according to documents seen by the BBC.
The jails have been told they need to find accommodation by August for 440 more prisoners, in total.
This figure represents about 0.5% of the prison population of 85,410. On average, the 40 prisons affected will have to find space for an additional 11 inmates each.
Mr Hardwick told BBC Radio 4's Today programme: "The situation is extremely serious and I am very concerned".
He said the system was not coping, and warned that because of staff shortages, men were locked up together for 23 hours a day, causing "huge tension".
He said extra resources were needed or the prison population had to be reduced.
"This is a political and policy failure - this is not the fault of staff", Mr Hardwick said.
The problem had arisen because "demands on the system have completely outstripped the resources available to them", he added.
Justice Secretary Chris Grayling said he disagreed with Mr Hardwick's comments.
He told Today there are currently 1,000 spare prison places, he said, promising 2,000 new ones by April.
He said there had been a recent increase in prisoners which could be linked to prosecutions for historic sex offences.
Analysis: Danny Shaw, Home Affairs Correspondent, BBC News
Nick Hardwick is a mild-mannered chap. Since his appointment as chief inspector of prisons four years ago he has steered clear of controversy, avoiding direct criticism of government penal policy - a feature of one of his predecessors, Lord Ramsbotham.
So his unexpectedly blunt warning about prison conditions and assertion that ministers are to blame are all the more likely to cause alarm in Whitehall.
Mr Hardwick's concerns about the long, hot summer echo those of staff at the sharp end: Governors and officers.
More prisoners crammed into cells and less time for education and exercise increase tensions.
Add to that tighter rules on day release after a series of dangerous offenders absconded and there's a real risk of disorder.
The justice secretary said the latest measures were to prepare for a possible further increase in the prison population over the summer.
"I am taking sensible precautions now to make sure that we are capable of expanding capacity to meet that", he said.
He defended the government's policy of replacing "old, Victorian prisons" and said overcrowding would mean that "a few hundred prisoners more will have to share a cell over the next few weeks".
It comes after a Prisons Minister, Jeremy Wright, said in a Parliamentary answer last month that an extra 412 places had been commissioned at six private jails since February.
The private prisons - Altcourse, Dovegate, Forest Bank, Lowdham Grange, Peterborough and Parc - are all already officially classed as overcrowded because they are holding more inmates than they were designed for.
The BBC's Danny Shaw said that, on top of those 412 places, a further 188 places have also been commissioned at private jails.
There is no information so far on where the extra 188 places are to be found.
Labour has pointed the finger at the government's decision to close some prisons, saying the space shortage was the result of ministerial incompetence.
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.
"The idea of prisons being able to rehabilitate offenders so they stop reoffending in these circumstances is a joke."
The Prison Reform Trust called on politicians of both sides to "wake up to the damage they are doing", while the Prison Officers Association described the development as a "fiasco".