The further fall in the shares, which were down more than 10 per cent at $5.74 at 1pm in New York, prompted Citi's bankers and customer managers to redouble efforts to prevent a flight of capital, according to people close to the situation. However, they added that Citi had not experienced unusual movements in deposits, corporate balances or prime brokerage accounts.

The billionaire Saudi prince, who invested in Citicorp in 1991 when it was in dire financial straits, said he would raise his stake in Citi from the current 4 per cent to 5 per cent. At Citi's current market value of about $30bn, the investment is worth some $300m.

In a statement, the prince said Citi's shares were “dramatically undervalued”. He expressed “full and complete support” for Citi's chief executive Virkam Pandit, who is under growing pressure to deal with the financial services group's crisis.