Microsoft's Surface RT tablet sold 1 million units in the fourth quarter, fewer than expected, according to a UBS analyst.

The analyst, Brent Thill, had expected 2 million Surface RT tablets to sell. In a note to investors, he said that customers are preferring iPads over Surface tablets.

Part of Microsoft's problem is that the software maker sold the Surface RT only in its own stores through much of the fourth quarter, he said, and didn't add other stores, including Best Buy and Staples, until mid-December.

In mid-December, Microsoft claimed it was increasing Surface RT production and called the public's reaction to the tablet positive. Microsoft won't reveal sales numbers for the device.

UBS' lower sales figures came a few days after Samsung revealed it would not be releasing its own Windows RT tablet in the U.S., citing little demand and lack of public understanding of the Windows RT operating system, when compared to the full Windows 8 OS.

UBS's Thill said there is hope for sales to workers of the full Windows 8 Surface Pro tablet due to launch later this month for $899 for a 64 GB version and $999 for a 128 GB version.