The watchdog says that the Treasury was “justified” in propping up the troubled sector to protect the wider financial system and that the final cost to the taxpayer will not be known for a number of years.
The study also revealed that the cost of financial advice to the Treasury from September 2007 will have reached £107m by April next year.
The report says that Credit Suisse could earns as much as £15.4m in fees for its crisis advice, while Deutsche Bank was also offered advisers on a contract of £200,000 a month for a year.
National Audit Office head Amyas Morse says it is difficult to imagine the scale of the consequences for both the economy and society had the banks been allowed to collapse.
He says: “As the crisis begins to subside, lessons must start to be learned. The authorities need to put formal arrangements in place to evaluate the effectiveness of the support provided to banks in order to inform future policy makers,”