Leopard, the newest version of OS X, was launched on 26 October and since then Apple claims to have sold or delivered more than two million copies of the software.

But a test of Leopard by Heise Security security expert Jurgen Schmidt found that the firewall in the updated software was set to off and allowed any and every incoming net connection.

Mr Schmidt also found that installing the software as an upgrade to a machine on which the firewall was turned on would lead to this protective software being turned off when that computer was re-started.

He also found that even when the firewall was re-activated it did not let users know about all the potentially vulnerable processes running on that machine.

Commenting for Heise Mike Barwise, editor of the site, said: "This as a serious problem: It provides a false sense of security, which is more dangerous than a lack of actual security that you know about."

Apple has yet to respond to a BBC request for comment on the Heise findings.

Mikko Hypponen, chief research officer at F-Secure, said: "Year after year, Macs continue to have these potential security problems.

"However, in practice they just don't seem to become real-world problems," he added. "The old wisdom still stands: if you want to avoid viruses and worms, get a Mac."