Duress in English law is a complete common law defence, operating in favour of those who commit crimes because they are forced or compelled to do so by the circumstances, or the threats of another. The doctrine arises not only in criminal law but also in civil law, where it is relevant to contract law and trusts law.

Contract law

Duress involves illegitimate threats. The common law long allowed a claim if duress was of a physical nature. So long as a threat is just one of the reasons a person enters an agreement, even if not the main reason, the agreement may be avoided. In Barton v Armstrong Mr Armstrong tried to "strong-arm" Mr Barton into paying him a large golden parachute to exit a business by getting his goons to make death threats to Barton's family. Even though Barton was tough, and would have probably done the payout regardless, he could avoid the agreement.

Only late in the 20th century was escape allowed if the threat involved illegitimate economic harm. A threat is always "illegitimate" if it is to do an unlawful act, such as breaking a contract knowing non-payment may push someone out of business. However, threatening to do a lawful act will usually not be illegitimate. In Pao On v Lau Yiu Long the Pao family threatened to not complete a share swap deal aimed at purchasing their company's building unless the Lau family agreed to change a guarantee agreement assuring the Paos would receive the rise in the swapped shares' prices when repurchased. The Privy Council advised that the Laus signing the guarantee agreement after the threat of non-completion of the main agreement was only a result of "commercial pressure", not economic duress. Just by observing the Laus' behaviour, and consideration of the situation before signing, there was no coercion amounting to a vitiation of consent. However, contrasting to cases involving business parties, the threat to do a lawful act will probably be duress if used against a vulnerable person.

An obvious case involving "lawful act duress" is blackmail. The blackmailer has to justify, not doing the lawful act they threaten, but against a person highly vulnerable to them, the demand of money.

Criminal law

This is an exception to the general principle of criminal law that those who choose to break the law are held responsible for the crimes that they commit. The rationale of the exception is that the choice is not wholly voluntary. The Law Commission (1977 at paras 2.44–2.46) recognised the logic that, if the defence was going to be allowed at all, it should be applied to all offences. But this recommendation has not been adopted because it is felt that, in the case of the most serious crimes such as murder, no threat to the defendant, however extreme, should excuse commission of the crime (Elliott; 1989). The defence is also open to abuse. Smith (1994 at p584) commented:

... duress is a unique defence in that it is so much more likely than any other to depend on assertions which are peculiarly difficult for the prosecution to investigate or subsequently to disprove.

This approach has been adopted by the judiciary, most notably by the House of Lords in R v H [2004] 2 WLR 335:

Defences that the accused has been set up and allegations of duress, which used to at one time to be rare, have multiplied. We wish to alert judges to the need to scrutinise applications for disclosure of details about informants with very great care.

The prosecution's difficulty was at one time the greater when the issue of duress had not been raised by the defence until the trial was under way. To counter these problems, the Law Commission (1993 at paras 33–34) recommended that the burden of proof be shifted to the defendant to establish duress on the balance of probabilities. Since then there has been no specific enactment relating to duress but Section 5 of the Criminal Procedure and Investigations Act 1996, c.25, provides that the defence must serve on the Court and the prosecution the nature of the accused's defence in general terms, and the matters in issue. This would seem to apply to the defence of duress, and in R v Tyrell and others 2004 EWCA Crim 3279, there had been a specific, although late, reliance upon the defence.

A rigorous analysis of the doctrine of duress is difficult because it is invariably reliant upon the  particular facts in a given case, and there is usually an overlap between duress and the defence of necessity. See, for example, comments by Lord Woolf CJ in R v Shayler at Para. 42.

History and reasoning
Duress as a defence has existed for many centuries and originates in common law—its extension to cover duress by circumstances did not occur until the 1990s, specifically the case of R v Pommell.

The defence of duress (by threat), according to Lord Bingham in R v Hasan, "excuses what would otherwise be criminal conduct" rather than justifies it. Bingham draws a distinction here with self-defence regarding between the moral status of the victim: in a case of self-defence, the victim has themselves made an aggressive or criminal act towards the defendant. In a duress case, there is no such relationship of prior aggression. Bingham notes at paragraph 19:

The threat
The law limits the nature of the threat that has to be placed on a person for them to qualify as being under duress. The threat must be one of death or serious personal injury, either to the defendant or to his immediate family or someone close to him. In R v Singh, the Court of Appeal held that a threat to expose the defendant's adultery would not be sufficient threat to overbear the will of an ordinary person.

The defendant must have a reasonable and genuinely held fear of death or serious harm, usually in the form of specific threats directed at the defendant, his immediate family or someone for whom he feels responsible. In R v Graham, the threat was immediately and directly made to the defendant. In the Australian case of  R v Hurley & Murray escaped criminals compelled H to dispose of two corpses by holding his wife hostage, such that the threats to her "would have been operative during the entire period of his absence" and "his only concern must have been for the safety of the woman".

Following R v Conway and R v Wright, (where the threat related in part to the defendant's boyfriend) the specimen direction of the Judicial Studies Board suggests that the threat must be directed, if not to the defendant or a member of his immediate family, to a person for whose safety the defendant would reasonably regard himself as responsible which, if strictly applied, would be consistent with the rationale of the duress exception.

Causal link
As seen in R v Cole 1994 Crim. LR 582, there must be a direct causal link between these threats and the defendant's decision to break the law. Thus, the defendant's normal inhibitions must be overwhelmed by their belief in the efficacy of the threat. Following R v Safi (2003) it is held that the defendant need only prove he reasonably and genuinely believed there was a threat, i.e. the test is both subjective and objective in that the defendant's will must actually have been overwhelmed by the threat, and a reasonable person of average courage may also have felt compelled to act the same way under direction.

Immediacy
The circumstances in which the threats were made must also have offered no reasonable opportunity for evasive action. But, taken together, the questions of causation and immediacy have created a weakness in the limitations placed on the defence. In R v Hudson and Taylor 1971 2 QB 202, two young women who had witnessed a serious assault were intimidated and refused to identify the attacker in court. They were charged with perjury but allowed a conditional discharge. The court was prepared to allow leniency because these women lived in a community where physical retaliation for co-operating with the police was routine, and they had no reasonable means, given their age, experience, and lack of physical strength, of avoiding the implementation of the threat. This weakened the requirement that the threat's implementation must be so imminent that the defendant had to decide in that moment whether to break the law, and it has given rise to cases such as R v Cole 1994 Crim. LR 582 in which a man robbed several building societies to avoid the threats of a debt collector. Simon Brown LJ. at p583 held that the peril relied on to support the plea of necessity lacked imminence, and the degree of directness and immediacy required of a sufficient nexus between the suggested peril and the offence charged. The one making the threats had not nominated the crimes to be committed by the defendant. He had simply indicated that he wanted the defendant to repay the debt which would not necessarily involve the commission of an offence.

The characteristics of the reasonable person
In R v Bowen [1996] 2 Cr. App. R. 157, the Court of Appeal held person with a low IQ, short of mental impairment or mental defectiveness, was not necessarily less courageous or less able to withstand threats and pressure than an ordinary person. The relevant test (laid down in R v Graham 1982 1 AER 801) had two elements:

Stuart-Smith LJ gave comprehensive guidance as to which characteristics might be relevant in the jury's consideration: age, sex, physical disability or recognised mental illness might limit a person's ability to act in self-defence, but the fact that the defendant was more vulnerable, timid or susceptible to threats than a normal person were not characteristics of the reasonable person. Also excluded would be self-induced incapacity due to drunkenness or drug-taking (R v Flatt 1996 Crim LR 576).
