Richard Tilt, director general of the service, said that the number of inmates was increasing so rapidly that they could fill a new prison every three weeks.
Mr Tilt told an audience of penal workers and campaigners that by last Friday, the prison population in England and Wales stood at 56,150 - despite the exit of 541 prisoners in the early release debacle - and was growing at the rate of 1,000 a month.
Speaking at the Howard League's annual conference in Oxford yesterday he said that he could not guarantee that the much-hated process of "slopping out" would not make a return. The end of the practice, which was achieved in April, was one of the key recommendations made by Lord Woolf in 1991 following the 1989 riots in Strangeways jail in Manchester.
He added that with money for new buildings severely restricted, accommodating the massive influx meant "a serious risk" and by early next year the service would have to resort to holding them, at huge expense, in police cells.
Mr Tilt said that a short-term projection drawn up in May suggested that the population of the 136 prisons in England and Wales would hit 60,000 within a year.
He told the conference that at the start of next year, with the traditional increase in the numbers of people being sentenced to custody following the courts' Christmas holidays, the service would be under a particular strain.
He said: "I don't how we will get through that period next year. There's a serious risk of having to resort to police cells - a huge setback."
The most recent projections do not take into account the impact of tough sentencing proposals to be included in a new crime Bill this autumn which could add up to 15,000 to the population total.
Mr Tilt also suggested that it would be "extraordinarily difficult" for the service to afford the security improvements recommended in last year's Learmont report into the breakout from Parkhurst on the Isle of Wight.
He warned that the combination of the rising population and budget cuts would imperil rehabilitation programmes in prisons.
Frances Crook, director of the Howard League, blamed the rapid rise in prisoner numbers on the hard-hitting law-and-order rhetoric of politicians, particularly that of Home Secretary Michael Howard.
She said: "We now have good management within the system but they cannot manage a system under these circumstances.
"It's time for a new Home Secretary. The country cannot afford the lunacies of current policies."Reuse content