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Passing off is a common law tort which can be used to enforce unregistered trademark rights. The tort of passing off protects the goodwill of a trader from a misrepresentation that causes damage to goodwill.
The law of passing off prevents one person from misrepresenting his/her goods or services as being the goods and services of the claimant, and also prevents one person from holding out his or her goods or services as having some association or connection with the plaintiff when this is not true.
Passing off and trademark law 
A cause of action for passing off is a form of intellectual property enforcement against the unauthorised use of a mark which is considered to be similar to another party's registered or unregistered trademarks, particularly where an action for trademark infringement based on a registered trade mark is unlikely to be successful (due to the differences between the registered trademark and the unregistered mark). Passing off is a form of common law, whereas statutory law such as the United Kingdom Trade Marks Act 1994 provides for enforcement of registered trademarks through infringement proceedings.
Passing off and the law of registered trademarks deal with overlapping factual situations, but deal with them in different ways. Passing off does not confer monopoly rights to any names, marks, get-up or other indicia. It does not recognize them as property in its own right.
Instead, the law of passing off is designed to prevent misrepresentation in the course of trade to the public, for example, that there is some sort of association between the business of defendant and that of the claimant. Another example of passing off is where the defendant does something so that the public is misled into thinking the activity is associated with the claimant, and as a result the claimant suffers some damage, under the law of passing off it may be possible for the claimant to initiate action against the defendant.
Required elements 
There are three elements, often referred to as the Classic Trinity, in the tort which must be fulfilled. In Reckitt & Colman Products Ltd v Borden Inc  1 All E.R. 873 Lord Oliver reduced the five guidelines laid out by Lord Diplock in Erven Warnick B V v J Townend & Sons (Hull) Ltd  AC 731, 742 (HL) (the Advocaat Case) to three elements:
- Goodwill owned by a trader
- Damage to goodwill
Plaintiffs have the burden of proving goodwill in its goods/services, get-up of goods, brand, mark and/or itself per se.
The Plaintiff also has the burden of proof to show false representation (intentional or otherwise) to the public to have them believe that goods/services of Defendant are that of the Plaintiff; thus, there must be some connection between Plaintiff’s and Defendant’s goods/services/trade. They must show likelihood and/or actual deception/confusion in the public. Deception/confusion, however, does not consider a ‘moron in a hurry’. (See Morning Star Cooperative Society v Express Newspapers Limited  and Newsweek Inc. v. British Broadcasting Corp.,  R.P.C. 441 by Lord Denning.) It is the Court's duty to decide similarity/identity of the marks/goods/services the criterion of which often fall under three elements: aural, visual and conceptual similarity (often applied in trade marks infringement cases).
In relation to the element of damage to goodwill, there may be loss/diversion of trade or dilution of goodwill. The Plaintiff need not prove actual or special damage; real and tangible probability of damage is sufficient. This damage should however be reasonably foreseeable. It is insufficient to simply show likelihood/actual deception and/or confusion.
Ultimately, the Court must use common sense in determining the case, based on evidence and judicial discretion, and not witnesses.
Disclaimers may be insufficient to avoid passing-off or cause of action (See Asprey and Garrard v. WRA (Guns) Ltd  FSR 30.) However, this was expressed in the dicta of the decision.
Extended passing off 
One of the instances where passing off is actionable is the extended form of passing off, where a defendant's misrepresentation as to the particular quality of a product or services causes harm to the plaintiff's goodwill. An example of this is Erven Warnink v J Townsend & Sons (Hull) Ltd  AC 731, in which the makers of advocaat sued a manufacturer of a drink similar but not identical to advocaat, but which was successfully marketed as being advocaat.
The extended form of passing off is used by celebrities as a means of enforcing their personality rights in common law jurisdictions. Common law jurisdictions (with the exception of Jamaica) do not recognise personality rights as rights of property. Accordingly, celebrities whose images or names have been used can successfully sue if there is a representation that a product or service is being endorsed or sponsored by the celebrity or that the use of the likeness of the celebrity was authorised when this is not true.
Reverse passing off 
Another variety, somewhat rarer is so-called 'reverse passing off'. This occurs where the defendant markets the plaintiff's product as being the defendant's product (see John Roberts Powers School v Tessensohn  FSR 947). It will be recalled that orthodox passing off entails the defendant representing that his product is the plaintiff's product. In many cases, reverse passing off can be explained under the ordinary rules: for example where a defendant may represent that he or she made goods which were in fact made by the plaintiff so as to pass off his own business as a branch of the plaintiff's.
See also 
- Dastar Corp. v. Twentieth Century Fox Film Corp. — lawsuit over "reverse passing off"
- Perry v Truefitt
- Private label
- White-label product
- Satyam Infoway Ltd. v. Sifynet Solutions Pvt. Ltd.- Indian case on passing off in domain names
The Law of Passing-Off by Christopher Wadlow, 3 Rev Ed edition (November 23, 2005)