The BBC has learned that the UBS trader being questioned on suspicion of unauthorised trading alerted the bank himself.
The BBC's business editor, Robert Peston, says UBS's internal controls did not pick up the huge loss allegedly generated by its trader Kweku Adoboli. He says Mr Adoboli told UBS that he had engaged in unauthorised trades.
Robert Peston discusses why this will add to concerns that investment banks aren't capable of controlling the huge risks that their traders take.
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