UK banks were trading in the red on Thursday after Exane BNP Paribas speculated that lenders may need to raise more capital to meet regulatory requirements.The Prudential Regulation Authority (PRA), due to make its final statement in December, could introduce additional capital buffers on top of current regulation, Exane said. "Quite simply, if the PRA adopts its proposals in full we believe the UK banks will need to have fully loaded equity tier-1 ratios of at least 13% over time." This means that some £100bn of equity may be required from across the UK banking sector, based on the current size of lenders' balance sheets.Analysts believe that Lloyds (rated 'outperform') is the only UK bank that is likely to reach a 13% fully loaded equity ratio by 2015.Barclays and Standard Chartered are also rated 'outperform', while HSBC and RBS are given a 'neutral' rating."The banks will almost certainly be given time to build such capital and there is the potential for substantial liability management exercises. But it could delay dividend payments and depress return on tangible equity below investor expectations. BC