Online banking customers are being warned to watch out for a new type of virus that infects computers unbeknown to the user, and can lead to hackers wiping out their account.

Unlike other viruses that are activated by the user opening a spam e-mail, this virus is started by a trojan already embedded in the computer.

The trojan, which finds its way on to the computer through either an infected e-mail or by downloading itself while the user is looking at certain pornographic websites, is activated by the typing of a particular word.

The warning was issued yesterday by Bank of Ireland, though the virus that can affect all websites, according to Cormac O'Byrne, head of group information security at Bank of Ireland.

He said four of the bank's customers, one who was online in Vietnam, have reported the virus, which is thought to have originated in Brazil. However, none of the bank's customers has lost money.

"The geography of either the hacker or the bank customer is totally irrelevant," he said, adding that the only way of avoiding the virus is to ensure that your anti-virus software is as up to date as possible.

One of the four people to report the virus, had updated his software a month prior to its discovery and it was still able to get through.

Mr O'Byrne said the actions of hackers are becoming more and more sophisticated.

"They have realised it's easier to attack customers than it is to attack the bank as there are millions of customers who don't know about the threat and a few banks who are working to reduce the threat," he said.

Up to date figures on the impact of internet fraud on Irish banks aren't available, though cases are rising, according to Una Dillon, head of card services at the Irish Payment Services Association (IPSA).

Last year, internet fraud accounted for less than 1 per cent of the overall €9 million lost to fraud, according to IPSA. "Internet fraud is definitely on the increase," she said.