By Dominic Casciani BBC News Online

This week, more people were in the UK's prisons than ever before. What is behind this record?
On Friday 26 July 2002, the prison population in England and Wales reached an all-time high of 71,723. It is the culmination of a decade of growth in prison numbers under Conservative and Labour home secretaries.

Ten years ago, the prison population was declining as courts used more community-based punishments.

But that soon changed as home secretaries followed up rhetoric of being "tough" on crime by asking the courts to jail more people.

Government figures show that by 1999 there were 24,000 more people being sent to prison than ten years before, despite no real change in the number of adults being found guilty of indictable offences.

The increased population doesn't come cheap. Each prisoner costs the taxpayer approximately �27,500 a year to keep in jail and there is a 10-year �2.7bn prison building plan.

Historical rise in prison populations

Historically, the UK's prison population rose throughout the 20th Century. The most significant increases began after 1951 when the courts began sending more people to jail.

The rise in the number of prisoners halted in 1981 and began to fall, particularly because of the introduction of fixed-penalties and police cautioning for some first-time offences.

Today, there are 124 prisoners for every 100,000 people in England and Wales, the second highest rate among western European countries. The United States has the highest rate of imprisonment - 702 prisoners for every 100,000 people.

The home secretary factor

But what's behind the modern rise in the number of prisoners? Firstly, the government's 2001 Halliday Report which investigated sentencing concluded that the toughening up of sentencing did not mean that there were more serious crimes being committed.

This graph shows the decisions that have led to rises over the last decade - each number relates to a key piece of legislation and is explained below:

One: The last piece of legislation which significantly lowered prison populations came in 1992 (piloted by Kenneth Baker but enacted under Kenneth Clarke). This introduced early release measures and encourage the courts to make wider use of community-based sentencing.

Two and three: But ministers soon changed their mind. Kenneth Clarke introduced new legislation in 1993, partially reversing the 1991 act, to allow courts to take into account previous convictions when determining sentences. In practical terms this meant that repeat offenders would face longer sentences. Soon Michael Howard was at the Home Office helm and he doubled to two years the maximum sentence for juvenile offenders.

Four: Just before Labour came to power, Michael Howard's final piece of legislation introduced automatic life for some sex and violent offenders and a mandatory three years for a third burglary conviction. The Home Office predicts that England and Wales needs 5,000 extra prison places by 2010 to deal with this alone.

Five: Since Labour came to power, there has been a raft of changes influencing sentencing policy and it is perhaps too soon to predict their effect.

The former home secretary Jack Straw extended the use of electronic tagging, a move aimed at cutting the number in prison cells. But he also introduced other measures to toughen up sentencing for offences he described as the worst anti-social crimes, such as burglary.

Penal reformers also say that the speeches or statements by home secretaries influence the courts. The Howard League for Penal Reform has monitored prison numbers since David Blunkett became home secretary in 2001. It says that numbers being jailing remained constant until Mr Blunkett made a series of tough speeches earlier this year. Since then, jailings have risen by up to 500 a week.

Prisoners sentenced to up to six months

One of the major reasons for the rise in the prison population under both Labour and the Conservatives has been the increase in the use of shorter sentences. In 1990, just under 14,000 adults were given sentences of six months or less. Ten years later that figure had almost tripled.

Ministers say that these sentences tackle some of the worst anti-social crimes such as burglary by young men. But the Halliday report said that short sentences give the authorities little chance to tackle offending.

Young offenders in prison

The largest increases in prison numbers has been seen among young offenders (those aged between 15 and 20) as home secretaries have acted to lock-up those involved in drugs-related crime.

Between 1980 and 1993 the number of young offenders in prison fell by a half. Between 1999 and 2000 it jumped by a third.

One of the major reasons for this increase is the 1994 change in the law which introduced longer sentences for offenders aged 15 to 17.

By the middle of 2000, a third of young offenders were serving sentences of between 18 months and three years, mostly for burglary or theft.