BRITAIN'S Serious Fraud Office (SFO) is considering a criminal investigation into options trading losses at investment bank NatWest Markets after a report showed a £90 million sterling deficit had been deliberately concealed for two years.

The investment banking arm of National Westminster Bank admitted weakness in its control procedures had contributed to the loss in interest rate options trading. Five senior staff have left as a result, to add to last week's resignation of chief executive, Mr Martin Owen. It said the losses were deliberately hidden over two years by the creation of false profits and unauthorised transfers between different trading books.

"The losses... went undiscovered because of deliberate concealment and weaknesses in the operations and internal controls in this area," the bank said in a statement." The independent price checking regime. . . did not have sufficiently robust procedures for procuring external quotations and checking the pricing of options," it added.

It added that trader Mr Kyriacos Papoois, who left NatWest in December 1996, covered up losses and created false profits over two years.

The SFO is expected to decide within weeks whether it will open the inquiry, which would serve to prolong NatWest's embarrassment over the affair.

Britain's financial markets watchdog, the Securities and Futures Authority, is also considering the report before deciding whether NatWest Markets or any individuals will be disciplined.