Ulster Bank customers need to be careful to avoid further financial woes by being targeted by fake "phishing" emails purporting to be from the boss of their banking group.
Action Fraud said a scam email to NatWest customers, the sister bank in the RBS Group also affected by the IT issues, 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 National Fraud Authority government agency warned.
Alan Woodward, a professor of computing, said: "This shows how on-the-ball these opportunistic criminals are. Some people are going to fall for it, especially if they are desperate."