"(1) Under English law the court may restrain a defendant over whom it has personal jurisdiction from instituting or continuing proceedings in a foreign court when it is necessary in the interests of justice to do. (2) It is too narrow to say that such an injunction may be granted only on the grounds of vexation or oppression, but, where a matter is justiciable in England and a foreign court, the party seeking an anti-suit injunction must generally show that proceeding before the foreign court is or would be vexatious or oppressive. (3) The courts have refrained from attempting a comprehensive definition of vexation or oppression, but in order to establish that proceeding in a foreign court is or would be vexatious or oppressive on grounds of forum non conveniens, it is generally necessary to show that (a) England is clearly the more appropriate forum ("the natural forum"), and (b) justice requires that the claimant in the foreign court should be restrained from proceeding there. (4) If the English court considers England to be the natural forum and can see no legitimate personal or juridical advantage in the claimant in the foreign proceedings being allowed to pursue them, it does not automatically follow that an anti-suit injunction should be granted. For that would be to overlook the important restraining influence of considerations of comity. (5) An anti-suit injunction always requires caution because by definition it involves interference with the process or potential process of a foreign court. An injunction to enforce an exclusive jurisdiction clause governed by English law is not regarded as a breach of comity, because it merely requires a party to honour his contract. In other cases, the principle of comity requires the court to recognise that, in deciding questions of weight to be attached to various factors, different judges operating under different legal systems with different legal policies may legitimately arrive at different answers, without occasioning a breach of customary international law or manifest injustice, and that in such circumstances it is not for an English court to arrogate to itself the decision how a foreign court should determine the matter. The stronger the connection of the foreign court with the parties and the subject matter of the dispute, the stronger the argument against intervention. (6) The prosecution of parallel proceedings in different jurisdictions is undesirable but not necessarily vexatious or oppressive."
“33. First, the English court must have personal jurisdiction over the Respondent in respect of the dispute. If the English court has jurisdiction over the substance of a dispute to which the Respondent is a party, then it will ordinarily have personal jurisdiction over the Respondent; Masri v CCIC (No 3) [2009] 2 W 669. Usually, an injunction is sought ancillary to existing or pending proceedings and the requirement is easily satisfied. Relief cannot be granted unless valid service on the Respondent can be effected: Airbus Industrie GIE v Patel[1999] 1 AC 119 . Thus, a person outside the jurisdiction may nevertheless fall within the personal jurisdiction of the court if s/he can be served and is a party to substantive existing or pending proceedings. 34. Second, "the English forum should have a sufficient interest in, or connection with, the matter in question to justify the direct interference with the foreign court which an anti-suit injunction entails"; Airbus Industrie GIE v Patel[1999] 1 AC 119 . This will usually require an inquiry into the nature of the substantive proceedings. 35. Third, there must be an appropriate ground for obtaining relief. The applicant must demonstrate some form of unconscionable conduct on the part of the Respondent which justifies the injunction being granted. Commonly this is found in contractual applications where the parties agree an exclusive jurisdiction for resolving disputes, but one party then brings a claim in a country other than the contractual forum. In non-contractual applications, examples include restraining a subsequent foreign action proceeding in parallel with an action established in the English courts: hence the source of the Hemain injunction. The applicant must demonstrate that the bringing or continuing of those foreign proceedings is unconscionable (which can include oppressive or vexatious behaviour); an example is evidence of bad faith where the Respondent is exerting extreme pressure on the Applicant, as in Cadre SA v Astra Asigurari SA[2006] 1 Lloyds Rep 560 . 36. Fourth and finally, if all of the above are satisfied, the court must then exercise a discretion whether or not to grant an anti-suit injunction; Star Reefers v JFC Group[2012] EWCA Civ 14 . In so doing the court will have regard to all the circumstances which include the facts upon which the application is based, the connections with each jurisdiction, the nature of the substantive proceedings both in this jurisdiction and in the foreign jurisdiction, the principles of judicial comity, the circumstance in which the foreign proceedings are brought, the balance of prejudice to each party depending upon whether the injunctive relief is or is not granted, and any other relevant matters. In the case of children, the exercise would surely also consider their welfare.”