World leaders at the Group of 20 meeting this week should force banks to build up their reserves substantially to avoid another acute financial crisis, a leading association of regulatory experts said. Lurking behind the appeal from the European Shadow Financial Regulatory Committee, a panel of academics and former regulators, is a fear that the political momentum for deep-seated reform may be waning as the financial crisis ebbs in intensity. Although a recent meeting of G-20 finance ministers in London discussed ideas for increasing the reserves that banks must hold against losses, there was little sense of urgency.