Twelve million new zombie computers have been hijacked by cyber-criminals since January, according to a report by anti-virus software company McAfee.

The security vendor said there has been a 50 per cent increase in the number of computers hijacked without their owners' knowledge since 2008. This number is likely to be realistically much higher than McAfee is able to determine alone.

The United States is now home to the largest percentage of 'zombie' computers, hosting 18 per cent of all infected machines.

A zombie computer is a computer with Internet access that has been compromised by a hacker, a computer virus, or a trojan horse. A compromised machine is usually only one of many in a botnet -- which is a collection of software robots -- and will be used to perform malicious tasks under remote direction.

The majority of zombie computer owners are unaware that their system is being undermined in this way.

In its McAfee Threats Report, the company said: In this quarter, we detected nearly 12 million new IP addresses operating as 'zombies' computers under the control of spammers and others.

The figures come as a report from Deloitte said a global approach to cyber-security is needed to ensure the safety of the digital economy.

The stakes of not addressing cybersecurity now are high, and the risks of not doing it right are even higher: the wrong approach could foster isolation, the prospect of cyber-protectionism, and an inadequate balance between security and civil liberties, said Greg Pellegrino, Deloitte Touche Tohmatsu Global Public Sector Industry Leader in a statement.

Doing nothing is not an option, Deloitte's Greg Pellegrino told BBC News.