The mayor of Munich has revealed the cost of reversing its move to Linux from Windows will run into millions of euros in hardware alone.

Earlier this year the newly elected mayors of Munich raised the possibility the city could return to Windows, despite the authority having spent years switching to a Linux-based OS and free software.

Munich's adoption of open source software continues to generate debate, as one of best known examples of a large organisation swapping Microsoft for Linux on the desktop. Since Munich began its migration in 2004, a number of German authorities have followed a similar path.

No return to using Windows as the main desktop OS is planned, but the council is intending to conduct a study to see which operating systems and software packages - both proprietary and open source - best fit its needs. The audit would also take into account the work already carried out to move the council to free software.

Don't miss the latest: Munich: The journey from Windows to Linux and back again (free PDF)

Now in a response to Munich's Green Party the mayor Dieter Reiter has revealed the cost of returning to Windows.

Reiter said that moving to Windows 7 would require the council to replace all the PCs for its 14,000-plus staff, a move he said would cost €3.15 million. That figure did not include software licensing and infrastructure costs, which Reiter said could not be calculated without further planning. He said a move to Windows 8 would be far more costly.

Reiter said going back to Microsoft would mean writing off about €14m of work it had carried out to shift to Limux, OpenOffice and other free software. Work on project implementation, support, training, modifying systems, licensing of Limux-specific software, on setting up Limux and migrating from Microsoft Office would have to be shelved, he said.

He also revealed that the move to Limux had saved the council about €11m in licensing and hardware costs, as the Ubuntu-based Limux operating system was less demanding than if it had upgraded to a newer version of Windows.

In the response Reiter addressed reports that he had told Stadtbild magazine that he was a Microsoft fan, saying his personal beliefs had no bearing on the IT audit.

"Since my interview in Stadtbild in which I was portrayed as a Microsoft fan, I have received much correspondence about whether our IT can satisfactorily meet the needs of users at all times and whether it's powerful enough for a modern metropolitan authority.

"There are many aspects to this, one of which is the corresponding user satisfaction. It's not about my personal taste, nor my individual experiences with open source."

Suggestions the decision to hold an audit was driven by complaints from staff about the shift to open source were not substantiated in Reiter's response. He said a workplace questionnaire had generated responses from staff relating to a range of IT issues and not just the Limux OS.

He also addressed a question about the relative security and Windows and Linux-based OSes. Reiter referred to a recent study by the German national security agency BSI, which found that Linux had a higher number of vulnerabilities than Windows but a smaller proporition of critical ones. However, he added the comparison was open to interpretation.