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Quinn v. Leathem [1901] A.C. 495, is a case on economic tort and is an important case historically for British labour law. It concerns the tort of "conspiracy to injure".

The union in this case (the Belfast Journeymen Butchers' and Assistants' Association) had wanted to enforce a closed shop agreement against a meat producer (Leathem). They approach one of his customers (Munce) and told him that he should refuse to trade with Leathem unless Leathem enforced the closed shop. And they said that if Munce did not do as they wished, they would call a strike among Munce's own workers. Munce had been buying Leathem's beef for 20 years, though there had been no written contract about it, and none of Munce's workers had yet been induced to strike (break their contracts).

But the House of Lords held that there was a "conspiracy to injure", which consisted in the intention to cause harm to others. It is perfectly lawful for one person acting alone to attempt this, but if it is two or more, then it suddenly becomes unlawful, and liability in tort follows. "It is," said Lord Macnaghten at p. 510, "a violation of legal right to interfere with contractual relations recognised by law if there be no sufficient justification for the interference."