The source of the jurisdiction of the High Court to grant an injunction (whether interim or final) is s.37 of the Senior Courts Act 1981 and of the county court to do so iss.38 of the County Courts Act 1984. The jurisdiction is exercisable even where it interferes with the exercise of statutory powers expressly conferred on one of the parties (Coventry City Council v PGO  EWCA CIV 729).
An injunction may be granted in support of any legal or equitable right known to English law (including directly enforceable rights arising under English law). Such rights include, in addition to causes of action at common law or under statute, directly enforceable rights arising out of the treaties constituting the European Union and Directives issued by the Commission pursuant to the treaties (Garden Cottage Foods Ltd v Milk Marketing Board  A.C. 130; Argyll Group Plc v Distillers Co. Plc  1 C.M.L.R. 764; Cutsforth v Mansfield Inns Ltd  1 W.L.R. 558). An injunction may also be granted:
As an interim remedy under s.25(1) of the Civil Jurisdiction and Judgments Act 1982 (as amended by the Civil Jurisdiction and Judgments Act 1991 s.3 and Sch.2 paragraph 12 and the Civil Jurisdiction and Judgments Order 2001 (SI 2001/3929) made in pursuance of Council Regulation (EC) No. 44/2001, and extended by the Civil Jurisdiction and Judgments Act 1982 (interim Relief) Order 1997 (SI 1997/302)).
To restrain the commencement of threatened or intended and the continuation of actual proceedings before courts in England and Wales, or before foreign courts, or arbitration proceedings (Mann v Goldstein  1 W.L.R. 1091; Stonegate Securities Ltd v Gregory  Ch. 576—restraint of presentation of winding up petition; British Airways Board v Laker Airways Ltd  A.C. 58; The Sennar  1 W.L.R. 490, HL; The Angelic Grace  1 Lloyd’s Rep. 87; Bremer Vulcan v South India Shipping Corporation Ltd  A.C. 909 – arbitration).
The discretion to grant or withhold an injunction is exercised after a trial on settled principles.
An injunction to restrain a threatened breach of a clear negative obligation (such as a negative covenant in a lease) will be granted to a claimant almost as of right: Docherty v Allman (1878) 3 App. Cas 709. The reason is that the injunction amounts to an order for specific performance of a negative bargain between the parties. Sees also Araci v Fallon  EWCA Civ 668. In that case, a jockey had entered into an agreement with a racehorse owner that had placed him under a positive obligation to ride the owner’s horse when asked, and a negative obligation not to ride a rival horse. The court granted an interim injunction preventing him from breaching his obligations by riding a rival owner’s horse in the Epsom Derby.
In other cases, there are certain factors which are recognised as affecting the discretion to grant or withhold an injunction. In Shelfer v City of London Electric Lighting Co.  1 Ch. 287 at 322-323, A.L. Smith, L.J. said that an injunction may be refused if: (1) the injury to the plaintiff’s legal rights is small; and (2) it is one which is capable of being estimated in money; and (3) it is one which can adequately be compensated by a small money payment; and (4) the case is one in which it will be oppressive to the defendant to grant an injunction. These four factors are cumulative and all must be present before the court can order damages in lieu of an injunction (Jacklin v Chief Constable of West Yorkshire  EWCA CIV 181).
These principles are not to be treated as an exhaustive statement of the circumstances in which damages may be awarded instead of an injunction: Jaggard v Sawyer  1 W.L.R. 269 at 287. However, damages in lieu of an injunction should only be awarded in very exceptional circumstances (Watson v Croft Promo-Sport Ltd  EWCA CIV 15). The slightness of the damage may, if the defendant threatens to repeat his wrongful act indefinitely, be “the very reason why an injunction should be granted”: see John Trenberth Ltd v National Westminster Bank Ltd (1980) 39 P. & C.R. 104. Where title is not an issue, a landowner is prima facie entitled to an injunction to restrain a trespass even if the trespass did not or will not harm him. Only if the defendant can show an arguable case that he had a right to do what the plaintiff sought to prevent, should the court go on to consider the balance of convenience, the preservation of the status quo and the adequacy of damages as a remedy (Patel v WH Smith (Eziot) Ltd  1 W.L.R. 853).