"I can pretty much guarantee that that incident will not happen again because of the actions we've taken subsequently," RBS's Chief Administration Officer Simon McNamara said on a conference call.

RBS has invested 750 million pounds ($1.2 billion) to improve the resilience of its systems. Britain's financial regulators on Thursday fined the bank 56 million pounds for a system crash in 2012 that affected millions of customers.

(Reporting by Matt Scuffham; Editing by Simon Jessop)