The Justices' Clerks' Society has hit out at Government proposals to introduce minimum and mandatory sentences, saying they may lead to a regression to a more primitive system of law.
The society said it had "grave misgivings over the use of minimum sentences, both in principle and in practice".
The Penal Affairs Consortium agreed, and said that the proposals were "the worst assault on the principles of justice this century".
Chairman Paul Cavadino said: "The Crime Bill is a desperate bid for electoral popularity at any costs, even though this means sacrificing the fundamental principles of justice."
Both organisations believe that mandatory sentences will lead to fewer guilty pleas and more plea-bargaining, leading to guilty pleas for less serious offences.
The Crime (Sentences) Bill, published last Friday, aims to introduce mandatory life sentences for twice-convicted sex or violent offenders and minimum sentences for third convictions of domestic burglary and serious drug dealing.
Home Secretary Michael Howard intends the Bill to be made law by Easter but it is unlikely to have an easy passage through the Lords.
Speaking on the Today programme last Friday, Lord Donaldson, former Master of the Rolls, said he had grave reservations about mandatory sentences, which he said could lead to gross injustice.