Police in London have charged a trader employed by UBS with fraud, in connection with the loss of more than $US2 billion because of unauthorised trading.
Kweku Adoboli, 31, was remanded in custody in the City of London Magistrates Court until 22 September. He was arrested on Thursday morning and is accused of carrying out unauthorised deals.
UBS said it may report an overall loss for the third quarter because of the deals.
The BBC has learned that the Swiss investment bank believes that the losses were accumulated in a large number of small trades over many months, not in one big deal.
Moody's says it is reviewing UBS's rating, focusing on ''ongoing weaknesses'' in its risk management.
Standard & Poor's suggested it was considering lowering the bank's A+ rating.
UBS lost £35 billion in the 2007-08 banking crisis and had to be bailed out by Swiss taxpayers.