To suggest that we have tried to keep them [disturbances] secret, or that they have amounted to riots, is quite wrong

Prisons chief Martin Narey

He partly blamed overcrowding for the disturbances and reiterated his call for fewer short-term custodial sentences.

The news of the unrest comes as the Prison Service is considering using a further ship to house inmates because jails are over-full.

The prison population in England and Wales is at a record level of 71,000.

Ringleaders punished

Speaking on BBC Radio 4's Today programme, Mr Narey described the disturbances as "worrying" but promised that the ringleaders would spend longer in prison.

More than 80 cells were damaged in the disturbances.

Numbers in jail are at official capacity

Mr Narey said: "To suggest that we have tried to keep them secret, or that they have amounted to riots, is quite wrong."

He warned that fewer people should be sent to prison.

"Overcrowding is not helping because every prison is very, very full at the moment.

"This is why it is important that sentencers listen to the home secretary and stop sending short-sentence people to prison when there are adequate community alternatives available," he said.

Barricades

No-one was seriously injured in the three separate disturbances.

At Guys Marsh, Dorset, last Thursday 30 prisoners barricaded themselves in and damaged fixtures, a Prison Service spokeswoman said.

One prison officer needed hospital treatment for bruising.

At Lindholme jail in Dorset on Monday, 25 prisoners on a wing refused to return to their cells.

The stand-off, classed by the prison service as "indiscipline", ended eight hours later when they surrendered.

At Ranby in Nottinghamshire, on Tuesday evening, 48 prisoners took over their residential unit.

Extra prison officers were drafted before inmates surrendered at 0430 BST.

Prison ships

A Prison Service spokeswoman told BBC News Online that it had been exploring the possibility of using another ship but had not entered into any negotiations yet.

It currently uses the Weare, at Portland in Dorset.

The Times newspaper had said officials had made inquiries about hiring three ships, each capable of housing 500 low-risk offenders.

Home Secretary David Blunkett has tried to ease overcrowding through early release under the electronic tagging scheme.

He has also called on judges and magistrates to be tough on dangerous offenders, but to jail people only when appropriate.

The chief inspector of prisons Anne Owers recently warned that the size of the prison population was compromising work to stop people re-offending.