The losses at the investment banking arm of the bank led to the resignation of NWM's chief executive, Martin Owen, and other senior staff.
The SFO, which investigates cases of serious or complex fraud, said that in the NatWest case the public interest would best be served by the matter being dealt with by markets regulator the Securities and Futures Authority. The SFA has the power to fine individuals and bar them from working in the financial markets. The SFO decided not to proceed after studying an internal report commissioned by NatWest and carried out by Coopers & Lybrand, the accountancy firm, and Linklaters & Paines, the law firm.
"The SFO has carefully considered the circumstances surrounding the losses incurred by NWM in the interest rate options business and has decided that an investigation with a view to a criminal prosecution is not justified," the SFO said.
It added that SFO director Rosalind Wright had taken into "account the nature of the transactions involved, which were highly complex, and the powers of the regulatory authorities in this area.
"She [Ms Wright] takes the view that the public interest in ... this case would be more appropriately satisfied by the matter being dealt with by regulation."
A spokesman for NatWest said yesterday: "We welcome the SFO's decision. There has been a lot of speculation whether they [the SFO] would do anything over recent weeks. We have co-operated fully with them and we are glad it has been sorted out, as far as the SFO is concerned."
The spokesman added that the SFA had been kept fully informed throughout NatWest's own investigations of the losses. "Whether the SFA will do anything else is up to them," he said.
The SFA said yesterday that it was looking at the NatWest report. "We shall be studying the role of a number of individuals named within it," said an SFA spokesman.
He said the regulator would "undoubtedly have to gather additional information".
The report by Coopers and Linklaters showed that the losses were deliberately hidden for over two years by the creation of false profits and unauthorised transfers between trading books at the bank.
The initial losses, in German mark and sterling interest rate options, were due to poor trading made worse by adverse market conditions but it said no personal gain was made and no client lost out.
The fact that the interest rate options market is esoteric and highly complex also meant that the facts of the case would have been very difficult to explain to a jury, according to legal observers.