This article presents a comparative analysis of the non-recognition and non-enforcement of foreign arbitral awards under Article V(1)(a) of the 1958 New York Convention within the two legal systems of England and the United States of America. The main research question is what similarities and differences exist between the approaches of the national courts of these two countries when they face the defences of incapacity of a party and invalidity of the arbitration agreement. The research adopts a descriptive-analytical and comparative method, based on legal literature and the case law of both countries. The findings show that the courts of both countries, in principle, assess the capacity of the parties at the time the arbitration agreement was concluded, although English courts also take capacity at the time of the award into account; moreover, in both systems the burden of proving the invalidity of the arbitration agreement lies with the party alleging it. However, English courts determine the law governing the arbitration agreement on the basis of the parties' choice and, failing such a choice, the law of the seat, whereas US courts tend to assess its validity under the law they themselves consider applicable. Given that Iran has also acceded to the New York Convention, an understanding of these approaches can inform the reform of Iran's domestic legislation and judicial practice, enhance the confidence of the international business community in Iran's legal system, and strengthen the position of international commercial arbitration in Iran.