The massive US bank, Citigroup, today warned investors that its eleven billion dollar losses linked to the sub prime mortgage market could become even bigger. Last night, its chairman and chief executive, Charles Prince, resigned, saying it was the "only honourable course" he could take. Shares in other US and UK banks fell on fears that similar write-offs could emerge across the banking system. With the credit crunch apparently entering a new phase, the IMF's chief economist has said that it will need to re-assess next year's world growth forecasts.