UK Criminal Law: Basic Principles

Posted by Catherine Robinson on

5-min read

The vast majority of the offences you will learn about will require an actus reus and mens rea. Generally, the actus reus and mens rea must coincide in point in time. This blog post shall take you through the components of actus reus and mens rea, including omissions, state of affairs offences, factual and legal causation, de minimis conduct, the thin skull rule, intervening acts, medical intervention, direct intent, oblique intent, subjective recklessness, negligence, transferred malice, strict liability offences, and the contemporaneity rule.


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Actus reus

What is actus reus?

Actus reus relates to the ‘guilty act’ performed by the defendant that results in an offence being committed. The defendant must act voluntarily, so their actions cannot be the result of a reflex. Devlin J gave the example of a driver who is attacked by a swarm of bees in Hill v Baxter. He stated that the driver would not be liable for the harm caused by his actions since they would not be voluntary. For this point, also see R v Mitchell and R v Larsonneur.

Can an omission constitute an actus reus?

In some circumstances, the actus reus can also be what the defendant fails to do (an omission). There is no general duty of care to act, however an omission can be sufficient for the actus reus if the defendant voluntarily assumed the duty to act, as was held in R v Stone and Dobinson. In this case, a woman suffering from anorexia nervosa moved in with her brother and his mistress. She was subsequently found dead in her bed in very bad conditions. Since the brother and mistress had voluntarily taken in the woman, their failure to adequately look after her was sufficient to constitute the actus reus and they were found liable for her death. Similarly, a duty to act may arise through the existence of a contractual, statutory, or official duty, per R v Pittwood.

State of affairs offences

Sometimes, a defendant can commit an offence without actively ‘doing’ something in the usual sense of the word ‘doing’. An example of this is s 1 of the Prevention of Crime Act 1953, which criminalises carrying an offensive weapon in public. The defendant need not do anything with the weapon to commit the offence, the mere possession of it is enough. Another example is drink driving (Duck v Peacock).


In situations where the actus reus does not directly result in the consequence that constitutes an offence, the prosecution must show that the actions of the defendant were:

  • the factual cause of the consequence, and
  • the legal cause of the consequence.

Both factual and legal causation must be proved, as was stated by the Supreme Court in R v Hughes.  

For factual causation, it must be proved that the consequence would not have happened ‘but for’ the actions of the defendant. This is called the ‘but for’ test and its operation can be seen in R v White. Here, the defendant put poison in the victim’s milk, but she later died of a heart attack. Since the poison didn’t kill the victim, the defendant was not guilty for her murder (but was liable for the attempt).

In order for a defendant to have legally caused the consequence, the consequence must be a natural and probable result of the defendant’s actions:

De minimis conduct

There must be ‘more than a slight or trifling link’ between the conduct and consequence, per R v Kimsey. The defendant’s conduct need not be the only cause, provided that it is an ‘operating’ and ‘substantial’ cause (see R v Smith, R v Benge, and R v Jordan).

Thin skull rule

The defendant will be liable for the full extent of the harm caused, even if the harm is greater than might normally be expected. In other words, the defendant must take the victim as they find them, even if the victim has personal attributes that mean that the harm suffered is worse than it typically would be. This is exemplified through the case of R v Blaue. Here, the victim was stabbed but refused a blood transfusion as she was a Jehovah’s witness. The thin skill rule was applied and the defendant was convicted of manslaughter.

Novus actus interveniens

Sometimes, an intervening act may break the chain of causation so the defendant won’t be liable for the full extent of the harm. The intervening act may be:

  • the act of a third party;
  • an act of the victim; or
  • a natural (yet unpredictable) event.

See R v Marjoram for a good example of a case where the relevance of this principle was discussed.

Medical intervention

Typically, medical intervention will not break the chain of causation unless it is ‘extraordinary and unusual’. See R v Cheshire and R v Jordan.

Mens rea

What is mens rea?

Mens rea is the mental aspect of committing an offence; it is the requirement of a ‘guilty mind’. There are different types of mens rea for different offences:

Direct intent

With this type of intent, the defendant must have made an active decision to act in a way that led to the consequence that makes up the offence, e.g. stabbing someone with the intended consequence of killing them. A key case for this is R v Mohan.

Oblique intent

For this type of intent, the consequence must be a virtual certainty of the defendant’s actions (objective) and the defendant must perceive this (subjective). See R v Woolin and R v Matthews and Alleyne.

Subjective recklessness

Here, the defendant must appreciate that there is a risk of bringing about the consequence that makes up the actus reus of an offence, but proceeds with their reckless conduct anyway. Offences that have subjective recklessness as the required mens rea are referred to as crimes of basic intent. See R v Cunningham.


Most offences do not allow negligence to make up the mens rea, however there are some exceptions such as gross negligence manslaughter (GNM), see R v Adomako.

What is transferred malice?

In some cases, mens rea may be transferred from the intended victim to the actual victim if the defendant misses their original target. This can be seen in R v Latimer, R v Mitchell and R v Gnango.

However, malice can only be transferred for the offence intended in the first place, per Lord Mustill in AG’s Reference (No 3 of 1994) and R v Pembliton.

What are strict liability offences?

These offences require no fault or mens rea element. For example, s 4 Road Traffic Act 1988 criminalises driving while under the influence of drugs regardless of the defendant’s intention.

The contemporaneity rule

Generally, the actus reus and mens rea must happen at the same time, although some cases might be regarded as a continuous act:

  • Thabo Meli v R: the defendants beat up the victim, intending to kill him but he didn’t die. The defendants thought he was dead and rolled him off a cliff, and he died from hypothermia. Here, there was sufficient mens rea for murder.
  • Fagan v MPC: the defendant accidentally drove onto a police officer’s foot but then intentionally stayed parked. There was sufficient mens rea for assault.



The information provided in this blog post is based on the research I carried out for my law degree which I completed in 2020. I accept no responsibility for errors or omissions. Legal principles and interpretations may change over time, and the content presented here may not reflect the most current developments in UK contract law. This information is intended for general informational purposes only and should not be considered as legal advice or relied upon as a substitute for professional legal counsel. For the most up-to-date and accurate legal information or advice, it is advisable to consult with a qualified legal professional who is knowledgeable about the latest legal developments and can provide guidance specific to your situation.


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