LONDON/SINGAPORE (Reuters) – HSBC said on Monday a demerger or spinoff of its Asian business risks huge one-off execution costs, higher taxes and ongoing running costs for the bank, in its most detailed rebuttal yet to calls from its largest shareholder to break up the bank. Sources said in April that Ping An Insurance Group Co of China Ltd had urged HSBC to explore strategic options such as spinning off its mainstay Asian business to unlock greater shareholder value. (Reporting by Lawrence White and Anshuman Daga; Editing by Muralikumar Anantharaman)