Microsoft has been reprimanded over misleading advertising by the Advertising Standards Authority (ASA).
The ASA said different hardware was used in the claims
The UK watchdog upheld complaints about a magazine advert which claimed that the open-source operating system Linux was more expensive than Windows.
Referring to research, it read: "Linux was found to be over 10 times more expensive than Windows Server 2003".
The ASA concluded that the comparison was misleading because the operating systems ran on different hardware.
Microsoft had said the Get the Facts ad campaign was intended to compare competing file-serving set-ups that met the same needs and were intended for the same purposes.
According to the software giant, a field test by independent analysts showed that Linux could be 10 times more expensive than Windows.
A graph used in the advert compared the cost in US dollars per megabit per second of a Linux image running on two z900 mainframe CPUs, with a Windows Server 2003 image running on two 900 MHz Intel Xeon CPUs.
But the ASA ruling said the hardware chosen for Linux was more expensive than it needed to be and could have influenced the outcome of the analysis.
Microsoft has been asked to change the advert.
Linux is an open-source operating system, which means it is essentially "free".
But many software companies use elements of it as the basis for programs they make commercially available.