Global financial regulators are considering creating several classes of systemically important financial institutions (SIFIs) in a rule change that could impose higher capital requirements on Australia's Big Four banks.On Friday, German business newspaper Handelsblatt reported that officials were considering creating "two to five" levels of systemic importance. One or more of these levels could include Australia's Big Four. The reported proposal for multiple levels of SIFI is part of a complex behind-the-scenes global regulatory battle over how SIFIs will be defined and regulated.Under Financial Stability Board rules agreed last November, tougher rules are to be imposed on a small group of SIFIs. In late 2010, this group was being referred to as "G-SIFIs" - that is, "globally systemically important financial institutions". It was expected to contain only global giants such as UBS, Citi, Credit Suisse, Deutsche Bank and BNP Paribas, which are a major presence in many key global markets. These giants could be required to hold equity equal to three percentage points above the basic 10.5 per cent capital-to-assets ratio required under Basel III.Large domestic players such as the Chinese and Australasian banks were not in this group because their businesses were mainly concentrated in their home markets. They were referred to as "local SIFIs".But, since November, a debate has emerged over whether, and how, to impose tougher rules on a wider group of SIFIs that could include some  "local SIFIs".Last week, Australian officials confirmed they were concerned that at its widest this group could include Australia's Big Four.Reports now suggest that depending on the outcome of this debate the Big Four could be required to hold capital equal to an additional one per cent of assets under the SIFI rules.The debate is believed to be taking place within the GHOs, the group of central bank Governors and Heads of Supervision that governs global prudential regulator the Basel Committee.Recent reports suggest France and Germany are leading attempts to widen the list of SIFIs affected by the rules; this would reduce the impact on their own national champions.  But US Federal Reserve supervision chief Pat Parkinson also wants a "gradated" approach to the SIFI issue, he told the Financial Times this month.Handelsblatt reported earlier this month that the group of SIFIs affected by the tougher capital requirements could now be as large as 30.Banking Day believes the debate is also looking at whether further groups of SIFIs should be hit with tougher capital requirements and other loss-absorbency measures soon after the initial group of "global SIFIs". The size of the additional capital requirement and other loss-absorbency measures for various groups of SIFIs also appears to be under discussion. Australia's regulators have consistently said that Australia's Big Four banks are already close to complying with the basic Basel III capital requirements.But an additional one per cent equity requirement would give the Big Four a more substantial capital-raising task over the next few years.As reported in Banking Day, Treasury's Jim Murphy said last week that the list of SIFIs should be restricted