Global sales of tablet computers are likely to slow this year, hurt in part by saturation and adoption of large-screen smartphones or "phablets," a market tracker said Thursday.

The research firm IDC said its updated forecast sees tablet sales up 12.1 percent this year, after a 51.8 percent expansion in 2013.

IDC said sales are likely to total around 245.4 million units this year.

"Two major issues are causing the tablet market to slow down. First, consumers are keeping their tablets, especially higher-cost models from major vendors, far longer than originally anticipated," said IDC analyst Tom Mainelli.

"And when they do buy a new one, they are often passing their existing tablet off to another member of the family."

The second factor is the rise of phablets -- smartphones with 5.5-inch and larger screens, the analyst said.

The emergence of phablets is "causing many people to second-guess tablet purchases as the larger screens on these phones are often adequate for tasks once reserved for tablets."

In the past year, IDC said, the phablet share of smartphone shipments has more than doubled, from 4.3 percent in the first quarter of 2013 to 10.5 percent in early 2014.

IDC said consumers are now looking at tablets and related devices with larger screens like Microsoft's 12-inch Surface Pro 3.

"The shift back toward larger screens will mark a welcome sea change for most vendors as the average selling price for these devices will remain roughly 50 percent higher than the average sub-8-inch device," said IDC's Jitesh Ubrani.

"Microsoft is also expected to benefit from this shift, as the share for Windows-based devices is expected to double between now and 2018."

A previous IDC report said sales of tablets including newly introduced convertible PCs totaled 50.4 million units in the first quarter of 2014.

That was just 3.9 percent higher than the same period a year earlier, and down 35.7 percent from the busy holiday season that included the fourth quarter of 2013.