Microsoft has eliminated four of eight previously announced security patches scheduled to be available to system administrators next "Patch Tuesday".

The software giant Friday amended a notice posted on its website just the day before announcing the scheduled eight patches.

Three of the bulletins removed addressed vulnerabilities in Microsoft Windows, the Office business software suite and Visual Studio, a software development programme, with a maximum severity rating of "important." The fourth one addressed a vulnerability in Microsoft Windows with a maximum severity rating of "critical".

Microsoft said it still plans to release the remaining four patches. Three of them affect Office and one affects Windows; each is rated "critical."

"Critical" is the highest level of urgency on Microsoft's ranking system, indicating that the vulnerabilties could be exploited to run malicious software on a PC with no action on the part of the victim.

Asked for an explanation of the sudden change, a company spokesman said: "There are many factors that impact the release of a security update, and every vulnerability presents its own unique challenges."

Microsoft releases security patches on the second Tuesday of each month, an event that has become known as "Patch Tuesday" to system administrators.