Magistrates reacted angrily yesterday to a call from the justice secretary, Jack Straw, to send fewer people to jail to ease pressure on prison places.
Cindy Barnett, chairman of the Magistrates' Association, said it would be "very unfortunate" if ministers tried to pressurise the courts over the sentences they handed down.
Her comments followed a warning by Straw on Thursday that the prison population had climbed to 82,000, outstripping official forecasts. In an interview with the Guardian, he urged the courts to consider using more non-custodial sentences, which had a better record of preventing re-offending than the short prison sentences which magistrates can impose.
Yesterday the number of prisoners in England and Wales rose to 82,068 inmates, almost 100 above the normal ceiling set by the Prison Service.
But Barnett said that the courts must have the freedom to impose sentences as they saw fit in individual cases.
"We are incredibly aware of the pressures on prison places at the moment. We don't use custody lightly, we use it when it is so serious that nothing else can be justified and we must make that individual decision ..." she told BBC Radio 4's World at One. "I think it would be very unfortunate if anything was said that is seen as pressure on individual sentencers."
She said that there were areas where the probation service lacked the resources to provide the sort of community sentences the courts wanted to hand down. "There are some areas of the country where, although courts wish to impose a community penalty of a particular sort - unpaid work for instance - there is not enough money left at the end of the financial year for that unpaid work order to be put into effect," she said. "We think that that is very wrong and that should be looked at as a matter of urgency."
Straw insisted that he was not seeking to dictate individual sentencing decisions but said that it was appropriate for ministers to raise wider sentencing issues.
"I am not saying to magistrates that you should do X rather than Y in an individual case. Magistrates have to exercise an appropriate and judicial discretion," he told The World at One.
"It is right, however, that there should be at large a national, local, regional conversation about the most appropriate sentences in given categories of cases."
A Prison Service spokesman said: "Although we have breached our usable operational capacity, there are spaces remaining in some parts of the prison estate and we are trying to make maximum use of these spaces where practicable."
Ministers were also accused of "panic measures" to ease the crisis yesterday, after announcing plans to free foreign criminals at an earlier stage of their sentence. Thousands of convicted offenders will be eligible for release and deportation from Britain 270 days before the halfway point of their sentence, rather than 135 days.
Shadow justice secretary Nick Herbert said: "The prison system is now in genuine crisis. Contrary to what Jack Straw implies, this is not the fault of magistrates, but the result of sheer incompetence by this government."