The prison population needs to be reduced if more jail riots are to be avoided, the chairman of the Parole Board has warned.
Nick Hardwick, an ex-chief inspector of prisons, said the Government could not just rely on recruiting more staff, but had to get the number of inmates down to a manageable level.
Speaking in the aftermath of a major riot at HMP Birmingham, Mr Hardwick said plans to recruit 2,500 extra prison staff did not go far enough.
"That's an important step, but the problem is, it's going to take time to get those staff recruited, trained and effective.
"We are not prepared to pay for the size of the prison population that we now have. So, the balance between the prison population and the number of staff we have got is now unworkable," he told BBC Radio Four's World At One.
Mr Hardwick said sensible measures could be taken to reduce the number of prisoners across England and Wales.
"It would be a mistake to take emergency, reckless measures around the prison population, but if you don't do things in a planned and sensible way, then further down the track they may be forced into a much more difficult position."
Mr Hardwick said the situation in the prison system was "grave".
"On the one level Birmingham was not, I think, on the list of prisons that people were most concerned about, but there is no doubt, if you look at the prison system as a whole, the situation is very grave.
"The levels of violence, and suicide, and self harm, are not merely increasing, but the rate at which they are increasing is accelerating, and we have now had a succession of very serious incidents that are unusual, and the fact that you now have this spate of them is a matter for the most serious concern.
"Successive ministers cannot say that they weren't warned about this. I, and others, have been warning about this for a number of years, and, so, the fact that we have reached this state now shouldn't come as a surprise."
Justice Secretary Liz Truss is to make a statement to the Commons on Monday on the Birmingham jail riot.