The proposed giant Titan jails could be dangerous and ministers have failed to explain why they would save money, a prisons watchdog has said.
The National Council of Independent Monitoring Boards said it had "fundamental doubts" about the idea.
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Three new Titan jails, each with up to 2,500 inmates, have been proposed in a bid to ease overcrowding.
The council criticised the government's consultation on the plan, describing it as a "fait accompli".
Trying to manage the jails would cause "major staff problems", it added.
The council said "there will be major and potentially dangerous consequences if services such as health and education are provided centrally, as it will be difficult to protect the most vulnerable prisoners from those who might cause them harm".
The council's president, Dr Peter Selby, said: "Most of our boards favour smaller units and have negative experience of large establishments and clustering of prisons to achieve efficiency, but at the cost of effective rehabilitation.
"We shall continue to emphasise and carry out our task of monitoring fairness and respect wherever people are imprisoned, and point out the major disadvantages of prisons of a size that present serious management problems."
In its response to the government's consultation, the council concluded that it had "considerable concerns" about developing Titan prisons.
Prison groups such as the Prison Reform Trust, Nacro and the Howard League for Penal Reform are against the proposals.
Various sites considered
The Ministry of Justice is searching for sites of at least 50 acres for jails of four or five storeys, costing an estimated Ј350m each.
The initial likely locations are London, the West Midlands and the North West.
The current jail-building programme has a target of a total of 96,000 prison places by 2014.
A Ministry of Justice spokeswoman said: "The consultation is still underway and the government welcomes the IMB's contribution to the debate. These new Titan prison complexes will not be warehouses - prisoners will be accommodated in small units capable of addressing their needs and offending behaviour.
"The complexes will also be situated close to those areas which generate the greatest volume of offenders, therefore ensuring that important family and other links can be maintained.
"They will represent value for money for the taxpayer and best support the chances of cutting reoffending by building in facilities aimed at rehabilitation."
(BBC)
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