In one of the most sophisticated cyber attacks, hackers have robbed nearly 675,000 pounds from about 3,000 online bank accounts in Britain.

The scam was carried out by using a computer virus -- described by experts as the most dangerous malware programme ever created -- that emptied the bank accounts while showing customers fake statements, the Daily Mail reported.

Internet security firm M86 Security, which discovered the scam, said the hackers stole a total of 675,000 pounds from the unnamed British bank.

"We've never seen such a sophisticated and dangerous threat. Always check your balance and have a good idea of what it is," the security firm said.

The latest virus is a variant of the Zeus trojan banking virus which first emerged 3 years ago and is called Zeus v3.

According to researchers at M86 Security, the virus attacked the systems when victims were browsing the net. It swiped the online banking ID of the customer and hijacked their online banking accounts.

The malware was designed to attack only those accounts, which had substantial balances, it said.

Bradley Anstis, M86 vice-president of technology strategy, said: "This is an extremely sophisticated version of the virus and it cannot be detected by traditional security software."

The company said it was the most-sophisticated and dangerous virus yet seen and advised online banking users to check their balances regularly and have a good idea of what it should be.

Meanwhile, British high street banks do not believe they have become victims of the cyber criminals.

A spokesman for HSBC said: "There are millions of viruses and other malicious software. We urge people to take basic measure to protect themselves from virus attacks.

"Any customer who is a victim of fraud will be reimbursed by HSBC."

However, M86 said it believed one high street bank was breached and failed to act quickly after warnings last month.

More than 100,000 PCs in Britain have been infected with other forms of the trojan virus.

According to Financial Fraud Action UK, about 59.7 million pound was lost to online banking fraud last year.

Another 440 million pounds was lost to credit card fraud.

And the problem is said to be on the rise, with criminals attacking banks' customers rather than the banks themselves as they are seen as softer targets, it said.

A Financial Fraud Action UK spokeswoman said: "The idea that criminals are targeting people by using malicious software or Trojans is nothing new.

"Bank systems are hard to attack so they're having to go through the easier link in the chain, which is the customers.

"They're hoping customers aren't taking security precautions. We've been seeing this for the last few years and we're constantly urging people to protect their computers to try to mitigate the risk of becoming a victim.