Government plans to build three huge "Titan" jails to tackle the overcrowding crisis were heavily criticised by a prison watchdog.
The National Council of Independent Monitoring Boards said it had "considerable concerns" about the project to build the 2,500-place prisons in England.
Council chairman Peter Selby said staff in the Titans would struggle to cope with the needs of different kinds of criminal in one prison.
The controversial prisons, far larger than existing jails, were announced by Justice Secretary Jack Straw last year.
The Council, which represents the voluntary bodies which oversee welfare in prisons, said the Government's consultation on the Titans contained no reference to the task of monitoring "fairness and respect for people in custody".
In its response to the consultation, the Council said: "That omission sends a message that verges on the sinister: not that ministers have abandoned their commitment to decency and fairness, but that when it comes to devising strategies for dealing with the numbers crisis and the other challenges facing the prison estate the monitoring task has simply disappeared off the agenda."
Dr Selby told the BBC Radio 4 Today programme that the money used to build the Titan prisons could be better spent on reducing crime and "community sanctions" rather than sending more people to jail.
He said: "We are concerned that the Government seems to be going back on Jack Straw's own statement that we cannot build ourselves out of the prison numbers crisis.
"We are not just dealing with overcrowding - though overcrowding is a serious problem in some prisons. We are also dealing with a system that is overwhelmed by numbers.
"The proposal to add 7,500 in these three massive prisons is going simply to add another set of places where regimes will suffer because they will have to accommodate the needs of several different kinds of prisoners who, at the moment, we expect to keep separate from one another."
