This research aims to compare the regulatory capital instruments for Islamic banking institutions (IBIs) - in particular the qualifying Additional Tier 1 (AT1) capital instruments - as defined by Basel III, Bank Negara Malaysia (BNM) and IFSB-15 (issued by the Islamic Financial Services Board). Principally, the research examines the Shari'ah issues, especially related to subordination, arising in equity-based contracts when used for structuring AT1 capital instruments. In particular, it examines the mudarabah sukuk issued by the Abu Dhabi Islamic Bank (ADIB) in 2012. The study finds that the most appropriate Shari'ah contract that would be suitable for structuring AT1 capital instruments would be musharakah. The present study is considered an original attempt in examining an under-researched topic relating to Basel III and its Shari'ah perspective. The study will be an important reference point to Islamic banks when structuring AT1 capital instruments.

Basel III has redefined the criteria for qualifying regulatory capital instruments. Banks have to maintain Common Equity Tier 1 (CET1) capital of at least 4.5% of Risk-Weighted Assets (RWA) and Tier 1 (T1) capital should be at least 6% of RWA at all times, while total capital (i.e., Tier 1 plus Tier 2) must be at least 8% of RWA at all times. T1 capital will absorb losses during going-concern - a situation where the bank is still solvent and continuing operation. Tier 2 (T2), on the other hand, refers to gone-concern capital, which will absorb further losses when the bank is facing financial distress and reaches the point of non-viability.

Basel III has redefined the criteria for regulatory capital instruments. Accordingly, Islamic banking institutions (IBIs) have to consider the issuance of instruments that would meet both the objectives of Basel III and Shari'ah requirements. This research particularly aims to compare the regulatory requirements for issuing Tier-2 (T2) capital instruments as defined by Basel III, Bank Negara Malaysia (BNM) and IFSB-15. In this regard, the research examines the Shari'ah issues related to subordination and conversion arising in exchange-based contracts (such as murabahah and ijarah sukuk) and equity-based contracts (such as mudarabah and wakalah sukuk). The study relies on library research to collect secondary data in the form of classical works of Islamic jurisprudence, analyses such work and links it with the present day regulatory requirements. The study finds that there are Shari'ah concerns over the use of exchange-based contracts. However, the use of convertible mudarabah and wakalah sukuk could be justified.