Credit Suisse shares tumble more than 20%, leading a plunge in European bank stocks

• Credit Suisse shares fell 21% Wednesday after its Saudi backers ruled out more investment.
• The embattled Swiss bank revealed "material weaknesses" in its reporting on Tuesday.
• Shares in the pan-European Stoxx 600 index also tumbled in Europe, leading to trading halts.

Credit Suisse shares tumbled more than 20% in pre-market trading on Wednesday after its biggest backer ruled out investing any more into the troubled Swiss bank.