Should we believe the crime figures? Confidence in the Government is now so low that few people are prepared to take any official figures on trust. If it were true that police recorded crime really had fallen by 9per cent and crime measured by the British Crime Survey really was down by a tenth, the Government would deserve a little praise.

But it is not just a question of trust. By increasing the prison population the Government has been doing the right thing. However, it feels rather shame-faced about that increase and rarely makes the connection between lower crime and more prisoners. But there are two reasons why increasing the prison population will reduce crime - incapacitation and deterrence. First, prison incapacitates offenders. When they are in jail they can't break into your house, steal your car or stab your teenage son. Secondly, if word gets around among criminals that there is a bigger risk of going to jail, it has a deterrent effect. The more certain the chances of punishment, the more criminals will think twice.

When Tony Blair took office in 1997 there were about 61,000 criminals in jail. The latest figure is 83,575. How many crimes would have been committed if those 22,000 additional offenders had been at large?

The best evidence comes from a Home Office survey in 2000. Offenders about to start a prison sentence were asked how many crimes they had committed in the previous 12 months. The average was 140 crimes a year and, for those on drugs, 257. The Government has been trying to limit prison to the most serious offenders and we know that the majority of prison inmates have a drug or alcohol problem. The average today is, therefore, likely to be nearer 257 crimes than 140.

If we take the lower figure, incapacitating 22,000 criminals who would have committed 140 crimes a year prevents more than three million crimes. If they were all drug users the figure would exceed five million.

Could the increase in the prison population from 2006-07 to 2007-08 explain the fall in crime over the same period? Police-recorded crime fell by 476,900 offences.

Between April 2007 and April 2008 the prison population increased by 1,843. If the annual offending rate was 140, then 258,000 crimes would have been prevented. If the additional prisoners were all serious offenders, as the Government claims, then 473,000 crimes would have been prevented.

Sheer coincidence? Despite its bashfulness about prison, the Government plainly does not think so. It plans to increase prison capacity to 96,000 by 2014, despite the squeamishness of Lord Hurd of Westwell in a letter to The Times yesterday.

When he was Home Secretary from 1985 to October 1989, Lord Hurd set out to reduce the prison population and presided over one of the most rapid increases in crime yet. There were 46,800 prisoners in 1985 rising to 50,000 in 1988 as judges responded to the crime wave. Instead of backing the judges, Lord Hurd cut the prison population so that it fell to 45,600 soon after he left.

Crime under the British Crime Survey rose from 12.4 million offences in 1985 to more than 14 million in 1989, and police records show an increase from 3.6 million in 1985 to 4.5 million in 1990. Some people never learn.

The Government has lost confidence in itself to such an extent that it does not know how to claim credit for an effective policy when it has one. It should be shouting aloud that “prison works”. Instead it talks of releasing prisoners early, puts pressure on judges to hand down lenient sentences and acts as if it thinks that criminals prepared to stick a knife in someone will be deterred by a visit to the local A&E.

But what about those crime figures? Should we find the 10 per cent fall reassuring? Crime is historically high at about ten times the rate in the 1950s. True, during the 1990s it got up to about 12 times the 1950s figure. A fall is a fall, but we have still got a long way to go and not just by comparison with more than 50 years ago.

Police records throughout Europe reveal that England and Wales had the second-highest crime rate out of the 37 countries in the 2006 European Sourcebook of Crime, compiled by an international team (including the Home Office) under the auspices of the Council of Europe. In 2003, at 11,241 crimes per 100,000 population, our rate was more than double the average of 4,736.

Moreover, the Government does not really believe its own figures. Today's crime figures mention that 2.7 million fraudulent transactions were recorded on UK-issued cards in 2007, an increase of a fifth in one year, according to the organisation that handles payments. But how much fraud appears in police figures?

Total fraud and forgery was down by 22 per from 199,700 in 2006-07 to 155,400 in 2007-08. The more you look into the figures the more one doubts them. But they are the best we have and should not diminish the credit due to the Government for pursuing a policy of “prison works”.

David Green is the director of the think-tank Civitas

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