Eight of the UK's biggest banks may have to find an extra £50bn of capital to buffer themselves under increasingly strict requirements from regulators, according to a report by accountancy giant KPMG.
This is because several indicators suggests the regulatory system is already moving past the Basel III agreements on capital requirements for banks, potentially resulting in a tighter 'Basel IV' set of standards.
KPMG's report points to the gold-plating of Basel III rules in the US and UK among other states, some countries already moving beyond the existing framework and requiring higher capital ratios, and "widespread concerns" among regulators and analysts over banks' internal modelling systems for risk-weighted assets which may be tightened in future.
"The pace of change and the desire by regulators to safeguard financial stability at all costs means that the regulatory timetable is getting far ahead of itself," said Giles Williams, partner in Financial Services at KPMG.
"The outlines of 'Basel 4' are already becoming visible, five years before the technical implementation deadline for Basel 3.
"Care needs to be taken that the banks are not being asked to do too much too soon."
Under Basel III, banks are expected to have a core tier 1 capital ratio of 7% by 2019. This means they must have a reserve of cash worth 7% of their assets' value, determined by risk weightings rather than at current market levels, to protect against future losses.
This regime is designed to protect investors as well as taxpayers, who had to bail out failing banks else risk the total collapse of the entire system.
The Bank of England cranked up the pressure on banks with a shortfall in July when it exposed a £27.1bn capital hole on five of the UK's largest institutions' balance sheets.
Taxpayer-backed RBS and Lloyds had the biggest capital shortfalls of the five.
RBS, which is 81% owned by the government after a financial crisis bailout, needs to find £13.6bn (€15.9bn, $21bn).
Lloyds, in which taxpayers have a 39% stake because of a bailout, needs another £8.6bn.
The other three are Barclays, which must find another £3bn; Co-operative Bank, with a £1.5bn shortfall; and Nationwide, which needs another £400m.
So far the five banks asked to address capital shortfalls have ratios of between 2% and 5%.