Usually, for passing off, there are three necessary elements. Firstly, it is necessary to show that a trade mark, has a reputation, or at least goodwill (this is sometimes referred to as "protectable goodwill"). Therefore passing off cannot be used to protect a trade mark which is new, with little reputation, or where no trade in the UK has taken place. Secondly, it is necessary to show that a misrepresentation has taken place or is likely. This requires that the trade mark complained of must be sufficiently similar to the trade mark of the claimant for a customer to be deceived. Thirdly, there must be a likelihood of damage as a result of the deception.
The existence of these three elements must be proved by evidence, so that a passing off action carries a high evidential burden (in other words you will have to work hard to win), but it nonetheless provides a possible remedy when, for example, there has been blatant copying of a get up, when or a trade mark has not been registered but has substantial use and a reputation.
The practical consequences of the law of passing off
Essentially, the law of passing off means that in choosing a trade mark, you should consider whether there are any earlier relevant registered trade marks (by searching trade mark registers) and also whether there are any relevant unregistered trade marks (by undertaking common law searches such as general internet searches). Just because a mark isn't registered, doesn't mean that you won't infringe by using a similar mark.
A second practical consequence is that if you have not registered your trade mark and find a competitor using a suspiciously similar mark, you may have rights on which you can take action.