The Crime (Sentences) Bill was born at the Conservative Party Conference in 1995. The slogan was: “If you don't want the time, don't do the crime.” The message from the Home Secretary, with vocal support from the tabloid press, was that prison deters, and more prison deters more.

The crime bill has two central proposals:

Real time sentencing – parole and early release will be abolished; instead prisoners will be able to earn a small good behaviour discount. Whereas now the court imposes, for example, a nine-year sentence for rape, of which the defendant serves six, when the bill comes into force the sentence passed will be six years, which the defendant will serve subject to any small good behaviour discount.

Mandatory minimum sentences – three years for persistent burglars, seven years for persistent drug dealers, and automatic life sentences for those convicted of a second serious violent or sexual offence (unless there are exceptional circumstances).

The bill contains less controversial proposals too: extension of supervision for sex offenders after release from prison, powers for courts to pass a prison sentence and at the same time order admission to hospital, and powers to impose community service orders and electronic tagging as alternatives to prison for fine defaulters.

The last proposal is welcome as a way of keeping minor offenders out of prison, but opposition to the major proposals has been strong, even unprecedented. Real time sentencing will replace the carefully considered system of early release (itself described as “honesty in sentencing”) put in place by the Criminal Justice Act 1991, when the Government's approach was that prison was an expensive way of making bad people worse.

As David Thomas, of the Institute of Criminology at Cambridge University, pointed out, the crime bill will mean abandoning sentencing practices and tariffs developed over decades by the courts, with the likelihood of confusion and inconsistency as a result.

The parole board has described the abolition of parole as a potentially dangerous mistake, with no assessment of the risk to the public. There has also been strong reaction to real time sentencing. Judges will get flak for the perception that they are going soft on crime; the result is likely to be an increase in sentence length, with a corresponding increase in what is becoming the highest prison population in Europe.

Mandatory minimum sentences have been tried in the US. Its experience is not encouraging. 'Three strikes and you're out' has been one of the factors creating a US prison population in excess of 1.5 million, five times the number incarcerated in 1973 and a rate five times that of most of Europe (rates per 100,000 – US: 519; England and Wales: 93). Between 1980 and 1986 violent crime in the US decreased 16 per cent; between 1986 and 1991 it increased by 15 per cent. The indications are that imprisonment has minimal impact on crime levels.

'Three strikes' policies have clogged the courts because no-one pleads guilty, and filled the prisons are filled to overflowing, so that prisoners not subject to minimum sentences (including violent offenders) have to be released. They have produced injustice, with, for example, minor drugs offenders serving the same sentences as international drug cartel leaders. They are inflexible even with an exceptional circumstances escape clause. As Lord Taylor said, judges must be free to fit the punishment to the crime if justice is to be done.

Recent years have seen violent swings in criminal justice policy and a surfeit of criminal justice Acts. If the latest proposals are enacted, a new government will have to live with the consequences and pick up the substantial bill. The Government suggests an increase of 11,000 in the prison population (currently about 57,000) over a period of 12 years; other estimates put the likely increase at 25,000 or 30,000. The latter would require 48 new prisons the size of Dartmoor's at a cost of £3bn to build and with annual running costs of £630 million. In his Strangeways report in 1990, Lord Woolf warned of the dangers of overcrowding in prisons. How quickly some people forget.

In the House of Lords in May, Lord Taylor mounted a devastating attack on the proposals. He was supported by peers from both sides of the House, including five former Home Office ministers. This week has seen the spectacle of two former Tory Home Secretaries condemning the proposals in the Commons. Of course, public anxiety about crime must be addressed, but if the proposals fail, as many concerned with the criminal justice system believe they will, public confidence will be further eroded. Let us hope the Government will think again before it is too late.

David Penry-Davey QC is chair of the Bar Council.