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.

"Even if you look at the recent instant messaging worms people are using different tools and technology. People have got smarter about email threats, but they have not translated these skills to instant messaging and other media."

Harkins explained that, while most computer users are reasonably virus-aware, new social engineering techniques are still catching them out.

Most people know that clicking on a URL, for example, can have serious security consequences, but users are still falling for instant messages asking them to do just that.