Security experts have warned of a substantial rise in the number and complexity of hacking attacks during the first half of 2005.

According to research commissioned by carrier AT&T, the volume of traditional email attachment viruses has fallen, but the speed at which new variants are appearing is increasing.

Other threats like phishing and spyware are also on the increase, the study warned.

"We have seen more attacks in the past six months than we have in the past couple of years," said Bill Archer, president of AT&T Europe.

"The diversity, frequency and - I hate to say it - innovation in the way that company assets are attacked are changing rapidly. You need to be constantly keeping on top of it."

Malcolm Harkins, director of information security at Intel, added: "In terms of what we've seen, phishing and spyware have increased a lot.

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