Legal basis: Common law and s.3 Criminal Law Act 1967

Definition:

A person may use reasonable force to:

• Defend themselves or another, or

• Prevent a crime or effect a lawful arrest.

Elements:

• Use of force – The defendant must genuinely believe force is necessary (R v Gladstone Williams [1984]).

• Reasonable force – Force used must be proportionate to the threat as the defendant believed it to be (R v Owino [1996]).

Key Points:

• The test is both subjective (belief in necessity) and objective (reasonableness of force).

• Mistaken belief can still allow the defence if honestly held, even if unreasonable (Williams (Gladstone)).

• The defendant need not wait to be attacked (R v Bird [1985]).

• Force is not reasonable if excessive (R v Clegg [1995]; R v Martin (Anthony) [2002]).

Effect: Complete acquittal if the jury finds the force reasonable in the circumstances as believed by the defendant.

Definition:

The defendant is compelled to commit a crime because of threats or circumstances that would overbear the will of an ordinary person.

There are two types:

• Duress by threats

• Duress by circumstances

(a) Duress by Threats

Applies when the defendant is forced to commit a crime due to threats of death or serious injury.

Elements: (R v Graham [1982]; R v Hasan [2005])

1. The defendant reasonably believed they or someone else faced a threat of death or serious injury.

2. The threat directly caused them to commit the crime.

3. A person of reasonable firmness would have acted the same way.

4. The defendant could not safely escape the situation.

5. The defendant was not at fault in exposing themselves to the threat (e.g., by joining a violent gang).

Limitations:

• Not a defence to murder, attempted murder, or treason (R v Howe [1987]; R v Gotts [1992]).

• Must be immediate or almost immediate threat.

(b) Duress by Circumstances

Similar to duress by threats, but arises from external danger rather than a direct human threat.

• R v Martin (Colin) [1989] – Defendant drove while disqualified to save his suicidal wife.

Effect: Complete acquittal (except for excluded offences).

Definition:

Necessity applies where the defendant commits an offence to prevent a greater harm. It is rarely accepted as a separate defence but overlaps with duress and self-defence.

Elements:

• The act was done to avoid inevitable and irreparable harm.

• The harm avoided was greater than the harm caused.

• The defendant acted reasonably and proportionately.

Cases:

• Re F (Mental Patient: Sterilisation) [1990] – Doctors lawfully sterilised a woman unable to consent, to prevent serious harm.

• Re A (Conjoined Twins) [2001] – Separation of twins permitted to save one life though it would end the other’s.

Limitations:

• Generally unavailable for homicide, except in extreme medical or moral necessity cases.

• Often treated as a justification rather than a true excuse.