Judges, magistrates and probation officers have not had sufficient time to train or study the provisions in today's Criminal Justice Act 1993.
Probation officers also say the Act will lead to the prison population rising by 4,000 by the end of the year - restoring Britain to the top of the European league for locking people up.
The new legislation stems from the Government's U-turn over its 1991 Criminal Justice Act, which sought to imprison violent offenders for longer, while diverting children and petty criminals from custody.
But two key features of what was at the time heralded 'the most forward- thinking piece of legislation to come out of the Home Office' turned it into a seven-month failed experiment. Judges and magistrates protested that their sentencing powers had been too stringently curtailed by provisions reducing the circumstances under which previous convictions could be taken into account. And unit fines, designed to be a more equitable means of imposing financial penalties, produced some ridiculous anomalies - in one case a pounds 1,200 fine for dropping a crisp packet.
Against a background of high profile juvenile and violent crime, there was a belief among the Tory party grass roots that their party of law and order had gone soft on crime. Michael Howard, the Home Secretary, said in response that he had managed to amend the 1991 legislation in 'record time' to ensure the punishment fitted the crime.
But yesterday Harry Fletcher, assistant general secretary of the National Association of Probation Officers - whose 7,000 members received their Home Office instructions detailing how the new sentencing rules will work just over a week ago - said: 'This is pure politics coming before common sense and justice.'
He said that before introducing the 1991 Act, the country's 28,000 magistrates, 10,000 probation staff and 1,000 judges had all undergone long consultation and training - and six months after the Act's implementation there was still confusion among sentencers.
'This time there has been no detailed discussion nor time for training. For the next three or four months there will be even more chaos in the courts.'
Joyce Rose, chairwoman of the 24,000-member Magistrates' Association, said her concerns centred only on the new system of financial penalties which will still take offenders' incomes into account but will not be tied to the mechanistic formula of unit fines.
JPs' pleas that the new fine system be delayed until November had won them only a month's reprieve until 20 September.
Laurence Cramp, secretary of the Justices Clerks' Society, said courts would learn as they went along.
'It's not the best way of dealing with it. But I am certain it can be done.'
However, he questioned whether prosecutors and defence lawyers would be familiar with the changes.