Tort law: Economic torts - Tortious interference + Civil conspiracy/collusion + Restraint of trade

Tortious interference, also known as intentional interference with contractual relations, in the common law of torts, occurs when one person intentionally damages someone else's contractual or business relationships with a third party causing economic harm. As an example, someone could use blackmail to induce a contractor into breaking a contract; they could threaten a supplier to prevent them from supplying goods or services to another party; or they could obstruct someone's ability to honor a contract with a client by deliberately refusing to deliver necessary goods.

A tort of negligent interference occurs when one party's negligence damages the contractual or business relationship between others, causing economic harm, such as, by blocking a waterway or causing a blackout that prevents the utility company from being able to uphold its existing contracts with consumers.

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A civil conspiracy or collusion is an agreement between two or more parties to deprive a third party of legal rights or deceive a third party to obtain an illegal objective. A conspiracy may also refer to a group of people who make an agreement to form a partnership in which each member becomes the agent or partner of every other member and engage in planning or agreeing to commit some act. It is not necessary that the conspirators be involved in all stages of planning or be aware of all details. Any voluntary agreement and some overt act by one conspirator in furtherance of the plan are the main elements necessary to prove a conspiracy. A conspiracy may exist whether legal means are used to accomplish illegal results, or illegal means used to accomplish something legal. "Even when no crime is involved, a civil action for conspiracy may be brought by the persons who were damaged."

In the law of tort, the legal elements necessary to establish a civil conspiracy are substantially the same as for establishing a criminal conspiracy, i.e. there is an agreement between two or more natural persons to break the law at some time in the future or to achieve a lawful aim by unlawful means. The criminal law often requires one of the conspirators to take an overt step to accomplish the illegal act to demonstrate the reality of their intention to break the law, whereas in a civil conspiracy, an overt act towards accomplishing the wrongful goal may not be required. Etymologically, the term comes from Latin con- "with, together", and spirare "to breathe".

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Restraints of trade is a common law doctrine relating to the enforceability of contractual restrictions on freedom to conduct business. It is a precursor of modern competition law. In an old leading case of Mitchel v Reynolds (1711) Lord Smith LC said,

it is the privilege of a trader in a free country, in all matters not contrary to law, to regulate his own mode of carrying it on according to his own discretion and choice. If the law has regulated or restrained his mode of doing this, the law must be obeyed. But no power short of the general law ought to restrain his free discretion.

A contractual undertaking not to trade is void and unenforceable against the promisor as contrary to the public policy of promoting trade, unless the restraint of trade is reasonable to protect the interest of the purchaser of a business. Restraints of trade can also appear in post-termination restrictive covenants in employment contracts.