Shares in Morgan Stanley plunged on Friday as investor confidence was shaken by a rating agency warning of a downgrade and speculation that the terms of its $9bn (£5.3bn) stake sale to Mitsubishi UFJ Group could be altered.

Morgan Stanley’s shares closed down 22 per cent to $9.73 in New York, reducing the bank’s market capitalisation temporarily to $10.8bn and raising questions about whether MUFG would go ahead with its plan to invest $9bn in the bank for a 20 per cent stake.