If courts keep jailing rioters and looters, jails will be full within a month, says governing body.
Eoin McLennan Murray, president of the Prison Governors Association (PGA), said ministers may be forced to consider early release schemes.
This could be seriously damaging for the Tory party, who criticised the last Labour government for releasing over 80,000 inmates up to 18 days early to deal with overcrowding.
The prison population has been increasing by over 100 a day over the last week, as courts have been working overtime to process rioting and looting cases.
Last night there was space for another 1,500 people in UK prisons, which have an 88,000 capacity.
McLennan Murray says that if the courts continue to jail people at this rate, prisons will be full within a month.
He said: “Compared with where we were 10 days ago we have seen a rapid rise in the population and if that continues at that rate we would be in trouble.”
He said sentencing trends had reduced before the riots but “if that reverses then we are back to the bad old days and not one we can build our way out of”.
A Ministry of Justice spokeswoman said: “We currently have enough prison places for those being sentenced to custody as a result of public disorder.
“There is substantial capacity in the prison system.”