Chief executives have been warned to be on their guard against a campaign of personalised spear phishing attacks. Reports surfaced last week of emails arriving with bogus subpoenas requesting the named chief executive to click on a link purporting to contain court documents.

The link actually leads to a plug-in that contains a Trojan with the ability to take over the victim's computer.

The reason this attack is so dangerous is that it is correctly addressed and identifies the chief executive by name.

European data security firm Norman said that the emails look very realistic and, unlike many other phishing attempts, use good grammar and spelling.

Trygve Aasland, chief executive at Norman, was one of the recipients. "This email appears legitimate and the technique is clever in that most people will want to discover the details of why and by whom they are being sued," he said.

"Fortunately I am very much aware of these attacks and we remained unaffected. But I can see how others may have been tricked into opening the link and installing the so-called plug in."