The defences of lawful authority and good reason are the two statutory routes out of a conviction for carrying an offensive weapon or a bladed article in a public place under the Prevention of Crime Act 1953 and Section 139 of the Criminal Justice Act 1988. Lawful authority is narrow and covers people armed as part of an official role. Good reason is broader, available to anyone, but judged objectively by the court and subject to a reverse burden: once the prosecution proves you had the item in a public place, you have to prove the defence on the balance of probabilities.1Legislation.gov.uk. Prevention of Crime Act 19532Legislation.gov.uk. Criminal Justice Act 1988, Section 139
The Offences These Defences Apply To
Both defences sit on top of a set of possession offences. The Prevention of Crime Act 1953 catches offensive weapons in public places, meaning items made for causing injury (knuckledusters, flick knives), items adapted for that purpose, and ordinary items the carrier intends to use as a weapon.3Sentencing Council. Knives and Offensive Weapons Section 139 of the Criminal Justice Act 1988 catches any article with a blade or sharp point in a public place regardless of intent, with an exemption for folding pocketknives whose cutting edge does not exceed three inches.2Legislation.gov.uk. Criminal Justice Act 1988, Section 139
Courts have treated “good reason” under the 1988 Act and “reasonable excuse” under the 1953 Act as functionally equivalent, so the same analysis applies to both statutes. Section 139A extends the possession offence to school and further education premises, with the same core defences plus an added “educational purposes” ground.4Legislation.gov.uk. Criminal Justice Act 1988, Section 139A
When Lawful Authority Applies
Lawful authority is reserved for people who carry weapons because the state has given them permission to do so as part of a defined role. Members of the armed forces carrying service weapons on duty, police officers carrying issued batons or firearms, and other officials whose statutory powers require them to be armed all fall inside it.1Legislation.gov.uk. Prevention of Crime Act 1953
The authority has to come from somewhere external. A private citizen who concludes, however sincerely, that they need to be armed does not have lawful authority. The authority must trace to a statutory provision or direct government authorisation, and it only runs while the person is actually performing the duties that call for the weapon. An off-duty soldier carrying a service weapon to a pub cannot rely on it. Nor can a security guard carrying something their role does not authorise.
Because the defence is tied to role and function, it rarely needs a long argument in court. Either the paperwork and duty status establish it or they don’t. For almost everyone else charged with carrying a weapon or blade, the question is good reason.
How Good Reason Is Judged
Good reason is an objective test. The court asks whether a reasonable person would consider the explanation legitimate, not whether the defendant personally thought it was. The reason needs to connect the item to a specific, practical purpose that explains why it was with that person at that time and in that place. Immediacy and proportionality matter: an explanation that works at 2pm on the way home from a job may not work at 2am outside a nightclub, even for the same person carrying the same item.
What Does Not Count
Two kinds of explanation reliably fail.
The first is self-defence. Carrying a weapon because you fear being attacked is not a good reason, and the courts have been clear that allowing it would defeat the purpose of the legislation. This holds even when the person can point to a genuine, articulable threat. The law’s answer to the fear of attack is not private armament.
The second is forgetfulness on its own. In R v Jolie, the Court of Appeal confirmed that simply having forgotten a knife was in your bag does not amount to a good reason. Forgetfulness combined with another legitimate reason can succeed: if you used a knife at work, placed it in your bag, and genuinely forgot it was there when you stopped on the way home, the combination of the original work purpose and the honest lapse can be enough. Whether it is enough on the facts is for the tribunal of fact to decide.
What Does Count
Section 139(5) of the Criminal Justice Act 1988 names three categories that count as good reasons for carrying a bladed article. They are not the only good reasons available, but they are the ones Parliament specifically identified.2Legislation.gov.uk. Criminal Justice Act 1988, Section 139
- Use at work. A chef travelling to a kitchen, a carpet fitter with a Stanley knife, a tradesperson with a utility blade. The protection attaches to the work activity, not the profession at large. A chef who finishes a shift and then carries knives into a nightclub has left the scope of the defence. The way the item is being transported should match the purpose.
- Religious reasons. The kirpan carried by baptised Sikhs is the clearest example. The exception is not confined to Sikhism, but the carrier must genuinely be observing the religious practice.
- National costume. The sgian-dubh worn with traditional Scottish Highland dress is the standard example. The person has to actually be wearing or engaged in the national costume at the relevant time.
None of these are blanket immunities. The court still looks at the specific circumstances. A ceremonial blade worn to a formal event as part of national dress is protected; the same blade carried days later on an unrelated errand in ordinary clothes is not.
On school and further education premises, Section 139A adds “educational purposes” to the list, which covers tools legitimately needed for lessons such as craft or science classes.4Legislation.gov.uk. Criminal Justice Act 1988, Section 139A The full menu of defences in that setting is good reason, lawful authority, use at work, educational purposes, religious reasons, and national costume.
Who Has to Prove What
The possession offences carry a reverse burden, which is unusual in English criminal law. The prosecution has to prove the first part: that you had the offensive weapon or bladed article in a public place (or, for Section 139A, on school or further education premises). Once that is established, the burden shifts to you to prove lawful authority or good reason.1Legislation.gov.uk. Prevention of Crime Act 1953
The standard you have to meet is the balance of probabilities, not beyond reasonable doubt. You need to show your explanation is more likely true than not. That is a lower bar than the prosecution’s, but it still requires evidence rather than assertion. Saying “I had a good reason” from the dock without any supporting detail rarely works. What strengthens the defence is concrete material: a receipt showing you had just bought the item, a rota or payslip showing the work shift the item relates to, a colleague who can confirm the professional need, photographs or messages that place the item in context.
Timing evidence tends to carry particular weight because good reason is so sensitive to context. Being able to show that the stop happened on a direct route between purchase and home, or between work and home, often matters more than the identity of the item itself.
How the Offensive Weapons Act 2019 Changed the Picture
The Offensive Weapons Act 2019 widened the ground on which these defences now have to operate.5Legislation.gov.uk. Offensive Weapons Act 2019 Certain dangerous weapons, including items such as zombie knives and knuckledusters, are now unlawful to possess even in private, closing the gap that previously allowed home possession. Delivery of bladed products to residential addresses is restricted, and delivery to anyone under 18 is an offence. Possession of a corrosive substance in a public place became a standalone offence. A new offence of threatening with an offensive weapon in a private place was created. And the prohibition on offensive weapons on school premises was extended to further education premises.
The practical consequence is that lawful authority and good reason now have to answer for a wider set of situations than they did a decade ago, including some that take place inside private homes. Conduct that fell into a grey area under the older framework is more clearly on the wrong side of the line today, and the defences are the same ones carrying the extra weight.