A software bug is leaving VMware customers unable to log in to virtualised servers.

The issue began early on Tuesday when users attempted to power up virtual systems running the company's ESX 3.5 software. The user is greeted with an error message indicating that the machine's 'power on' function failed due to an expired licence.

The company said the issue is due to a timeout mechanism that had been left on and set to expire on August 12. This, said the company, caused the system to lock out users and believe that the licence had expired.

The issue only affects systems that run ESX 3.5 Update 2 and ESXi 3.5 Update 2.

The timeout feature is often used by developers when distributing test builds to users in order to prevent them running and distributing test versions of the software indefinitely.

In a blog posting, VMware assured users that the issue was not a security risk and that the cause of the problem had been found.

"Several customers have been worried that their ESX systems had been compromised by an attack and that this was the cause for not booting," the company said.

"The problem that surfaced today is not related to exploitation of a security issue on ESX."

The company said it is in the process of producing a temporary "express patch" due to be released by 6pm on Tuesday evening US pacific time. Users can also temporarily work around the flaw by setting the system's date and time to an earlier day.

VMware hopes to have a permanent patch ready for release by noon pacific time (20:00 GMT) on Wednesday.