Hacking and computer viruses - ranging from Melissa to the Love Bug - will cost global businesses around $1.6tn this year, according to figures published this week.

The survey conducted by PricewaterhouseCoopers (PwC) of 4900 IT professionals across 30 nations found that this year alone 39,363 human years of productivity will be lost worldwide because of viruses.

In total, the bill this year to US firms with more than 1000 employees for viruses and computer hacking will amount to $266bn, or more than 2.5 per cent of the nation's gross domestic product. The price tag worldwide soars to $1.6tn, according to the PwC study.

However, Alex Shipp, antivirus technologist at ASP MessageLabs, said PwC's estimates are far too high, and joked that they probably represent lawyers' estimates of how much virus writers should be sued, rather than real economic impact.

"The real effects of viruses are not as much as everybody shouts about. That said, I know of two cases where clients of ours have picked up contracts because their competitors were hit by the Love Bug," said Shipp. "It's really difficult to estimate how much viruses cost."

Bruce Walton, corporate sales manager at antivirus company Command Software, pointed out that Microsoft was effectively closed for one day because of the Love Bug, but agreed that estimating the true cost of virus outbreaks is difficult.

John DiStefano, principal researcher on the study at Reality Research & Consulting, which assisted on the project, said the $266bn figure represents the impact of viruses on US businesses with more than 1000 employees, or about 50,000 firms.

"These are companies with infrastructures of IT professionals who are increasingly tracking the problem and can provide an accurate assessment of the scope of the issue. In reality, the true impact of viruses on US business, including medium-sized companies and small businesses, is much greater," said DiStefano.The key costs involved in correcting IT systems infected by a virus are found in lost productivity as a result of downtime for the computer systems, as well as lost sales opportunities, he added.

According to the study, commissioned by US magazine Information Week, technology professionals in the US this year will suffer system downtime of 3.24 per cent, while downtime rises to 3.28 per cent on a worldwide basis.