Which crimes are least likely to be solved?

In a perfect world, every crime would be solved and result in a just and proportionate penalty.

However, in reality, most go unsolved and many are not even reported to police. So what is going on?

In 2018-19, fewer than 8% of offences led to a suspect being charged or ordered to appear in court, and under 4% were dealt with outside court, by an on-the-spot fine, caution or community resolution, such as an apology or compensation.

When deciding if a suspect should be charged, police generally work with the Crown Prosecution Service (CPS), which prosecutes crimes in court on behalf of the state.

The two tests which must be met for the CPS to authorise charges are whether there is sufficient evidence for a realistic prospect of conviction and if a prosecution is in the public interest.

Charging rates - and overall detection levels - vary according to what type of crime it is.

Drugs offences are often discovered by the police through searches or raids, which is why charging rates are higher.

Most murders are detected because of the huge investigative effort that goes into solving them.

And the proportion of crimes that are "detected" - meaning they have been resolved in some way - has been falling continuously since 2015.

But offences such as robbery, where witnesses are needed, and sexual offences, which often revolve around the issue of consent, have far lower clear-up rates.

In England and Wales, we measure crime through the Crime Survey of England and Wales, and by counting offences that are reported to and recorded by police.

The Survey is seen as the better way of measuring long-term trends - its methods have remained consistent and it includes unreported offences.

Its results indicate an overall downward trend in offending over the past two decades, before stabilising in the past few years.

However, some crimes - such as murder, offences against businesses, and crimes against people aged under 16 - are not included in its main findings.

The Survey is also not regarded as being good at estimating less common crimes which cause the most harm, like stabbings and robbery.

Police data provides a more reliable measure of these crimes, which have shown a genuine increase.

Overall, the police figures show the number of offences logged by forces is up.

This is partly because more people feel comfortable approaching the police about under-reported crimes, such as sexual offences, and because police themselves are getting better at recording crime.

Police officers are central to the criminal justice system's smooth running as they:

The number of officers in the 43 England and Wales forces has fallen by about 20,000 since 2010, but the government is in the process of restoring those numbers by April 2023 in the most ambitious police recruitment drive in modern times.

There has also been a steep decline in the number of police community support officers and civilian staff, who provide vital back-up roles.

When officers are called to reports of a crime, they have several options, including resolving matters informally, issuing a warning, or making an arrest - which can lead to a charge.

But as officer numbers have dropped, arrests have gone down - for almost all crimes.

Detective work has been made more complex because of the mass of digital material from smartphones, computers and data storage devices.

A roughly 30% reduction in the CPS budget between 2011 and 2018 has also affected the volume of prosecutions and how quickly cases can be processed, though court success rates have improved.

Once a case goes to court, most crimes are dealt with by magistrates or district judges but serious offences are heard in the Crown Court, where defendants have the right to trial by jury.

Almost half of Magistrates Courts in England and Wales have been closed since 2010.

This is partly because of a government strategy to move some cases, such as low-level thefts and motoring offences, out of the court system, by extending the use of video links and virtual hearings conducted online.

If a defendant admits the offence they are accused of, or is found guilty after a trial, a magistrate or judge will determine the most appropriate sanction.

The maximum (and in some cases, minimum) penalty for each offence is set out in legislation, but the judge will also refer to Sentencing Council guidelines.

Most crimes are dealt with through fines or community sentences, administered by probation officers.

The probation system was partially privatised in 2014, but is now undergoing a further substantial overhaul to undo some of the changes.

Prison is usually reserved for serious offenders, persistent criminals and those who pose a risk to the public; it is intended to be used as a "last resort", but England and Wales have one of the highest rates of imprisonment in the Western world, with almost 83,800 people currently locked up.

Prisons, like other parts of the criminal justice system, also faced significant budget cuts, which led to the loss of more than 6,000 front-line prison staff between 2010 and 2016.

The reduction coincided with a rise in violence, drug-taking and self-harm.

Prison officer numbers are now on the increase and the government has promised to invest £2.5bn in 10,000 new places.

But the legacy of years of austerity is still being felt.

By Reevel Alderson, BBC Scotland Home Affairs correspondent

Crime and prosecution statistics are not directly comparable with England and Wales.

Crime has been on a downward trajectory over the past 27 years, although there was a slight increase in crimes reported to the police last year.

As with south of the border there is another measure of crime in Scotland - the annual Crime and Justice Survey.

This has also shown a decrease in recent years.

Police officer numbers in Scotland have remained broadly similar since 2013, although Police Scotland has announced plans to reduce its strength by around 400 officers by 2021 to meet budget constraints.

The number of crimes that are "cleared up" has remained high, but this is not the same as crimes being solved.

This cleared up figure reflects all cases where there is sufficient evidence to justify criminal proceedings.

If an accused person dies before the case comes to court, or flees the country, the crime is regarded as having been cleared up.

Similarly, Northern Ireland has not seen an increase in police recorded crime.

Police officer numbers have decreased since 2010, although at a slower rate than England and Wales.

Around 30% of recorded crimes result in some form of action.