1. The tort of conspiracy consits of two or more people's combining to injure another by the doing of some unlawful act. The tort is an action on the case, so actual loss must be inflicted.
Leading case: Gregory v. Duke of Brunswick 1843 - a group of barrackers hissed an actor off stage.

2. Especially: combining to harm business interests of another
In executing such agreements, the object of causing loss to the plaintiff' business interests must be predominant.
This was finally recognized in the first case and confirmed in the second one:
Crofter Hand Woven Harris Tweed v. Veitch 1942 - object was to further interests of the defdt.'s own.
Lonrho Ltd. v. Shell Petroleum Co. Ltd. 1982 - same as above.
Where the primary object is to lawfully further one's own interests, the is no claim:
Mogul Steamship Co. v. McGregor 1892 - defendant wanted to create a monopoly, which was still legal back then (see below for illegality of object).

3. Those are cases where an element of 'malice' renders wrong what would otherwise be lawful. But the element of combination is essential; so there are certain flaws in the law when one individual, however powerful, undertakes to maliciously harm the business interests of another.

4. Where the conspiracy is such as to do something tortious in itself, the predominant object of harming the plaintiff need not be present (although it usually is): Lonrho plc. v. Fayed 1992
This is true also of trade disputes (below).

5. Trade disputes have been shielded from claims in conspiracy and certain allied torts (Trade Disputes Act 1906; now in the Trade Union and Labour Relations (Consolidation) Act 1992): Agreements or combination by two or more persons to do or to procure the doing of an act in contemplation or futherance of a trade dispute shall not be actionable in tort, if the act is one which, if done without any such agreement or combination, would not be actionable in tort.
In conlclusion, trade disputes are privileged as fas as an element of 'malice' should make an agreement or a combination as derided above unlawful.