Automatism is an act done by the muscles without any control by the mind. It is a complete defence and the defendant is acquitted when found not guilty. This defence is available for defendants whose actus reus is done not voluntarily. And they are missing the required mens rea for the offence.

The cause of the automatism must be external, like; Sneezing, the effect of a drug or an attack by a swarm of bees. Case example - Hill v Baxter 1958. Exceptional stress can be an external factor which may cause automatism. It was shown in case of T 1990 where the defendant was allowed the defence due to post-traumatic stress.

Self-induced automatism is where the defendant knows that his conduct is likely to put him on an automatic state. Examples include a diabetic, who knows the risk of failing to eat after taking insulin. or a person who drinks alcohol after medication when he was told by his doctor that he must not take alcohol while on that medication. Case of bailey 1983 established that the defence applies different to specific intent offences and basic intent offences.

For specific intent offences a self-induced automatism can be a defence. This is because the defendant lacks the required mens rea for the offence. Case example - Bailey 1983.

The law is more complicated on basic intent and the main rule is that the defendant cannot use the defence of automatic state if he has brought the automatic state by being reckless. If defendant has been reckless in getting into a state of automatic, self-induced recklessness cannot be a defence. If the self-induced automatic state is caused through a drink or illegal drug or other intoxicating substance, the defendant cannot use the defence of automatism. Case example - DPP v Majewski 1977. If the defendant does not know that his actions are likely to lead to a self-induced automatic state in which he may commit an offence, he has not been reckless and can use the defence of automatism.