NatWest customers could face further financial woes as they are being targeted by fake “phishing” emails purporting to be from the boss of their banking group.
Action Fraud said a scam email claiming to be from RBS Group chief executive Stephen Hester apologises for the problems and says a "security upgrade" requires them to update their information.
If customers follow the web link on the email, they are taken to an "incredibly realistic" replica of NatWest's website, Action Fraud said.
It warned that criminals could use any details entered to steal money or commit identity fraud.
Anyone who receives such a scam email should not click on any links or attachments in it and should not try to contact the senders in any way, the body - run by the National Fraud Authority government agency - warned.
Alan Woodward, a professor of computing, said: "This shows how on-the-ball these opportunistic criminals are. Imagine not being able to access your bank account and then getting one of these...
"Given the number of NatWest customers and the volume of emails that the scammers send, some people are going to fall for it, especially if they are desperate."