LONDON, UK: Computer hackers have stolen at least 675,000 pounds from thousands of bank accounts in Britain using a malicious software, which expert believe the most dangerous virus ever created.

About 3,000 online banking customers have been victims of a computer virus attack that empties their accounts while showing them fake statements so the crime goes undetected.

Experts have described the attack using a "trojan" virus as the most sophisticated and dangerous malware programme ever created, Daily Mail reported.

The cyber criminals stole an estimated 675,000 pounds between July 5 and August 4 and the attack is still progressing, experts warn.

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

M86 Security said: "We've never seen such a sophisticated and dangerous threat. Always check your balance and have a good idea of what it is."

The scam was discovered after M86 gained access to the command-and-control server in Eastern Europe running the thefts.

It collects data such as passwords and even transfers money out of accounts automatically, but only after checking if there is at least 800 pound available.

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.

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 personal computers in Britain have been infected with other forms of the trojan virus.