5. Conspiracy
This is the 5th post in Law of Torts, Unit 4 — Business and Reputation Related Torts and Remedies. This post covers the tort of conspiracy, the wrong committed when two or more persons combine together to injure another, and the two distinct forms this combination can take.
The Problem This Topic Solves
An act that would be perfectly lawful if done by one person acting alone can become unlawful when several people combine together to do it, because a combination carries a special capacity to cause harm that an individual acting alone often lacks — coordinated pressure, collective economic power, and organised action can crush a plaintiff's trade or livelihood in a way that no single actor could manage independently. The law of torts responds to this reality through the tort of conspiracy, which asks not merely whether the individual acts of the defendants were lawful, but whether the combination itself, viewed as a whole, was directed towards injuring the plaintiff without lawful justification. Understanding when a combination crosses the line from legitimate collective action — such as ordinary trade competition or lawful trade union activity — into an actionable conspiracy is essential to correctly analysing disputes involving group boycotts, coordinated trade pressure, and organised interference with another's business.
Meaning and Essentials
The tort of conspiracy consists of a combination of two or more persons who agree to do an unlawful act, or a lawful act by unlawful means, and who, pursuant to that combination, actually cause damage to the plaintiff. Unlike most other torts, where a single wrongdoer can be liable, conspiracy by its very nature requires a plurality of actors acting in combination — a person cannot conspire alone. The essentials of the tort are, first, a combination or agreement between two or more persons; second, an overt act done in pursuance of that combination; and third, actual damage suffered by the plaintiff as a result. Unlike many other torts, conspiracy is not actionable per se — mere proof of a combination and an unlawful or injurious purpose is not enough; the plaintiff must go on to prove that real damage was actually caused by the conspiracy.
Two Forms of the Tort
The tort of conspiracy is traditionally divided into two distinct forms, distinguished by the nature of the means used by the combining parties.
Conspiracy to injure by lawful means arises where the combination uses only means that would be lawful if used by a single individual, but the combination is nevertheless held liable because the predominant purpose of the parties in combining was to injure the plaintiff, rather than to protect or advance any legitimate interest of their own. Since each individual act involved is lawful in itself, liability here turns entirely on the combined actors' state of mind — specifically, on what their predominant purpose was in acting together. If the defendants' true, dominant object was to further their own trade, business, or other legitimate interests, and any resulting injury to the plaintiff was merely incidental to that legitimate pursuit, no liability arises, however severely the plaintiff may in fact have suffered. It is only where the predominant purpose is to injure — where causing harm to the plaintiff is the real object of the combination, rather than a side effect of pursuing one's own interests — that this form of the tort is made out.
Conspiracy by unlawful means arises where the combination uses means that are independently unlawful — such as an act that is itself a crime, a tort, or a breach of contract — to injure the plaintiff. Because the means employed are already unlawful in themselves, this form of the tort does not require the plaintiff to prove that the defendants' predominant purpose was to injure; it is enough that the combination used unlawful means and that damage to the plaintiff resulted, even if the defendants' primary motive was to advance their own interests rather than simply to harm the plaintiff. The unlawfulness of the means used substitutes for, and makes unnecessary, any inquiry into the defendants' predominant purpose.
| Basis | Conspiracy by Lawful Means | Conspiracy by Unlawful Means |
|---|---|---|
| Means used | Each individual act is lawful on its own | The means themselves are independently unlawful (crime, tort, or breach of contract) |
| What must be proved | The combination's predominant purpose was to injure the plaintiff | Just the unlawful means and resulting damage — no predominant purpose needed |
| Illustration | Quinn v. Leathem — predominant purpose was to injure Leathem's trade | Inducing a third party to breach an existing contract with the plaintiff |
Quinn v. Leathem (1901) — House of Lords.
Facts: The plaintiff, Leathem, was a butcher who employed non-union labour in his business. A trade union, acting through its officials including the defendant Quinn, wanted Leathem to dismiss his non-union workers and employ only union men, and when Leathem refused, the union officials combined together to induce one of Leathem's principal customers, a meat wholesaler, to stop dealing with him — they did this by threatening the wholesaler's own employees with a strike if the wholesaler continued to purchase meat from Leathem. As a direct result, the wholesaler stopped buying from Leathem, causing him substantial loss of trade.
Holding: The House of Lords held the union officials liable in conspiracy. It held that a combination of two or more persons who wilfully cause damage to a person in his trade or business, without justification, is actionable, even though no single act by any one of the combiners would, standing alone, have been unlawful — provided the predominant purpose of the combination was to injure the plaintiff rather than to further any legitimate trade or business interest of the defendants themselves. On the facts, the predominant purpose of the union officials' combination was found to be to injure Leathem's business by coercing him over his employment practices, not to advance any genuine trade interest of their own, and Leathem was awarded damages. The House of Lords distinguished this outcome from the earlier decision in Mogul Steamship Co. v. McGregor, Gow & Co. (1892), where a combination of shipowners acting to protect and advance their own legitimate trade interests through ordinary competitive practices was held not liable in conspiracy, since their predominant purpose was to further their own business, not merely to injure a rival. A full standalone Case-Law post on this decision is available in this unit's Case-Law list.
Distinction from Criminal Conspiracy
The tort of civil conspiracy must be carefully distinguished from the offence of criminal conspiracy, even though both concern a combination of persons agreeing to pursue a common unlawful object. Criminal conspiracy is complete the moment the agreement to commit an unlawful act, or a lawful act by unlawful means, is made — no further act need follow, and no actual damage need be proved, since the crime lies in the agreement itself. The tort of civil conspiracy, by contrast, is not complete merely upon agreement; it requires an overt act done in pursuance of the agreement, and — critically — actual damage suffered by the plaintiff as a result. A combination that agrees to injure another but never in fact causes any real damage may amount to a criminal conspiracy, but gives rise to no civil cause of action in tort, since damage is an essential element of the civil wrong that has no counterpart requirement in the criminal offence.
| Point of Distinction | Civil Conspiracy (Tort) | Criminal Conspiracy |
|---|---|---|
| When complete | Only on proof of actual damage caused by an overt act | Complete on the agreement itself, before any overt act |
| Proof of damage | Essential — not actionable per se | Not required — the agreement itself is the offence |
| Object | Compensation to the injured plaintiff | Punishment of the conspirators by the State |
| Who may proceed | The injured plaintiff, in a civil suit | The State, through criminal prosecution |
- Conspiracy requires a combination of two or more persons — a single individual cannot commit this tort alone.
- Essentials: combination/agreement, an overt act pursuant to it, and actual damage suffered by the plaintiff — conspiracy is not actionable per se.
- Lawful means conspiracy is actionable only if the combination's predominant purpose was to injure the plaintiff, not to advance the defendants' own legitimate interests.
- Unlawful means conspiracy is actionable once independently unlawful means are used and damage results, without needing to prove a predominant purpose to injure.
- Quinn v. Leathem (1901) established liability where a combination's predominant purpose was to injure the plaintiff's trade, distinguishing this from combinations pursuing their own legitimate trade interests.
- Civil conspiracy requires proof of actual damage; criminal conspiracy is complete on the agreement alone, without any need to prove damage.
- Mogul Steamship Co. v. McGregor, Gow & Co. (1892) illustrates lawful-means conspiracy going the other way — a combination of competitors held not liable because their predominant purpose was to protect their own trade, not simply to injure a rival.
- Ordinary trade competition, even where it seriously damages a rival's business, is not conspiracy so long as the combination's predominant purpose is to advance the combiners' own legitimate interests.
- The same set of facts can sometimes give rise to both a criminal prosecution for conspiracy and a separate civil suit in tort, but the two proceed independently and require proof of different elements.
A Practical Example
Several wholesale grocers in a town, angered that a new retailer is undercutting their prices, agree among themselves to pressure their common suppliers into refusing to sell goods to the new retailer, purely to drive him out of business and eliminate the competition, rather than for any genuine trade advantage of their own. If the suppliers agree and refuse to supply the retailer purely because of this pressure, and the retailer's business suffers real financial loss as a result, the retailer has a valid claim in conspiracy by lawful means — each individual act (a supplier declining to sell to a particular customer) would be lawful if done independently, but the combination's predominant purpose was to injure the retailer rather than to advance the grocers' own legitimate trade interests. If, instead, the grocers had achieved the same result by inducing the suppliers to breach existing supply contracts already in place with the retailer, the retailer would have a claim in conspiracy by unlawful means, since inducing breach of contract is itself an independently unlawful act, and the grocers' predominant purpose would not even need to be proved.
Quick Revision Points
- Conspiracy = combination of two or more persons + an unlawful act, or a lawful act by unlawful means + actual damage to the plaintiff.
- Essentials: combination/agreement, overt act, actual damage — not actionable per se.
- Lawful means conspiracy: liability turns on the combination's predominant purpose being to injure the plaintiff, not to advance the defendants' own interests.
- Unlawful means conspiracy: liability arises once independently unlawful means are used and damage results, without proof of predominant purpose.
- Quinn v. Leathem (1901) — House of Lords; combination to injure a butcher's trade over his employment practices held actionable.
- Mogul Steamship Co. v. McGregor, Gow & Co. (1892) — contrasting illustration; combination held not liable where predominant purpose was to advance the defendants' own trade interests.
- Civil conspiracy (tort) requires proof of actual damage; criminal conspiracy is complete on the agreement alone.
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