The law on self-defence is plain, simple and reasonable. What’s less clear is why Jack Straw, the Secretary of State for Justice, has announced he wants it changed. He says that a key part of citizenship is knowing that “the system will stand up for you if you stand up for fellow citizens”. In fact, that’s exactly what the law does now.

As the law stands, if you hurt someone while defending yourself, or while stopping a crime, you won’t be prosecuted even if you kill, so long as what you did was reasonable in the circumstances. You’ll only be prosecuted if you have acted unreasonably. And reasonableness isn’t judged by the standards of behaviour at a vicar’s tea party, it’s judged by what someone in desperate circumstances would do.

The public – manifested in juries — can and has refused to convict people who have used violence to prevent crimes. In 1988, Ted Newberry, a 76-year-old from Ilkeston, Derbyshire, lay in wait on his allotment shed for an expected intruder, then shot a 12-bore gun at a Mark Revill when he tried to enter. Revill was badly injured and Mr Newberry was prosecuted on charges of wounding, but was acquitted by a jury.

That sort of decision has a long history. For example, between 1300 and 1348 homicide was the third most common offence prosecuted in England but there were frequent acquittals where householders had killed housebreakers. In 1604, in a dispute between Peter Semayne and Richard Gresham about what force could be used to defend a home, a judge said: “The house of everyone is to him as his castle and fortress.” The principle that an Englishman’s home is his castle has since echoed through the ages.

Guidance issued in 2005 by the Crown Prosecution Service and the Association of Chief Police Officers says that anyone can use reasonable force to protect themselves or others or to prevent crime. It couldn’t be plainer. It is based on the common law and section 3 of the Criminal Law Act 1967. A citizen isn’t expected to make fine judgments over the level of force used in the heat of the moment. The official advice says:

“So long as you only do what you honestly and instinctively believe is necessary in the heat of the moment, that would be the strongest evidence of you acting lawfully and in self-defence. This is still the case if you use something to hand as a weapon”

If you’re at home and under threat, you don’t have to wait to be attacked. You can strike first. But you shouldn’t be too pre-emptive. In the allotment shed case, although the jury acquitted Mr Newberry of the criminal wounding charge, the injured intruder won compensation for his injuries (the award was £12,000 reduced to £4,000 because of his contributory negligence) even though he’d been acting criminally.

The play of these principles in court has produced various results. Tony Martin, the Norfolk farmer, was convicted of murder in 1999 for shooting a young burglar in the back. On appeal, Martin’s conviction was reduced to manslaughter (on the grounds of diminished responsibility) and he was released in 2003.

In another case in 2000, David Summers was caught by the men into whose Peterborough flat he had broken. In their fearful defence they inflicted multiple injuries on him with a metal baseball bat. Sentencing Summers to a year in jail for burglary, Judge Hugh Mayor, QC, said he wouldn’t reduce the sentence on account of the injuries. He said: “They used reasonable force . . . You brought that on yourself and I have no sympathy for those who receive hurt while committing a crime.”

There are several cases where the intruder has been killed without there being any prosecution of the killer. But the law won’t turn a blind eye to wanton revenge. That would lead to violent anarchy. If, having knocked an intruder unconscious, you then battered him as a punishment, you’d be acting with gratuitous force and could be prosecuted.

In 2005, the Director of Public Prosecutions stated that during the previous 15 years (when the courts dealt with over 20 million crimes) there had only been 11 prosecutions against householders including one in which a burglar was tied up, thrown in a pit and set alight. Some people might say, “So what, it serves him right!” – but imagine what sort of society we’d descend into if that was our way of punishing crime. It would lurch towards those of Mad Max and 28 Days Later.

Results of a recent survey suggest that almost a third of householders in the UK keep items such as golf clubs and cricket bats under the bed ready to fight intruders. It’s worse in America, which has only five times the population of the UK but 20 times the number of homicides.

Professor Gary Slapper is Director of the Centre for Law at The Open University

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