Worst hit, according to a worldwide survey by IBM, are government departments, financial services, manufacturing and healthcare.

Of the 237 million security attacks in the first half of 2005, 137 million were aimed at these four areas.

Spam is becoming less attractive as criminals focus on fraud, identity theft and extortion.

This has meant a decrease in the ratio of spam to legitimate e-mail from 83% in January to 67% in June.

US most popular target

In the first half of the year, IBM's Global Business Security Index recorded 35 million phishing attacks, where criminals try to trick people into handing over confidential data.

Highly targeted and co-ordinated attacks on a specific organisation, known as spear phishing, increased more than ten-fold since January.

While spam seems to be on the decline, viruses are definitely not. According to IBM, the January average of one in every 52 e-mails infected with some sort of malicious security threat has risen to one in every 28 by June.

The index found that the US remains the most common source of attacks, with a total of 12 million over the six month period looked at.

New Zealand was, perhaps surprisingly, second with 1.2 million attacks, followed by China with one million.

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"To protect their critical data, infrastructure, brands and money IBM advises businesses to rethink how they protect their operations, business processes and governance structures," said John Lutz, general manger of the Financial Services Sector of IBM.

The IBM Global Business Security Index Report is a monthly report that assesses, potential network security threats based on the data and information collected by the company's 3,000 worldwide security experts and thousands of monitored devices.