Police have said a 21-year-old man is the first in the UK to be arrested for "phishing" - an e-mail scam which tricks people into giving bank details.
People are advised not to respond to e-mails asking for bank details
The National Hi-Tech Crime Unit said they arrested the man from Lytham St Anne's in Lancashire on Thursday.
Police investigated after Smile, the internet bank from The Co-operative Bank, reported the problem in March.
A spokeswoman for the National Hi-Tech Crime Unit said: "It's the first arrest in the UK for phishing."
She said the man, who is unemployed, had been released on bail until August while experts examined his computers.
He is not thought to be part of an organised crime gang behind scams in the UK, USA, Australia and New Zealand.
Detective Chief Superintendent Len Hynds, head of the National Hi-Tech Crime Unit, said they believed the suspect was a "copycat phisher".
Phishers pose as a bank to request personal details by e-mail as part of a bogus "security check".
They can then use the information to take cash from people's bank accounts.
Phil Garlick, director of operations at Smile, said: "The security of our customers' money is absolutely paramount to us.
"We regularly remind customers via secure messages and on our website not to respond to e-mails of this kind as we would never ask customers to send personal information about their Smile account to us in this way."
The Association for Payment Clearing Services (APACS) warned UK bank customers last month that they might be targeted by a new wave of scam e-mails.
In the past, customers of major UK banks such as Natwest, Barclays and Lloyds TSB have been targeted.