GENEVA — Online sales of apparel and accessories, along with computers and consumer electronics, are projected to continue to spearhead the double-digit expansion of U.S. retail e-commerce over the next four years, a report by research company eMarketer predicts.

In 2013, U.S. retail e-commerce sales are forecast to reach $259 billion, up 14.8 percent over last year’s total of $225.5 billion, the report said. Apparel and accessories, as well as computer and electronic sales combined will contribute 42.9 percent of total sales, and will edge up to 45.6 percent by 2016.

Emarketer estimates sales of online apparel and accessories this year will jump by $8.6 billion to $54.2 billion from last year’s figure of $45.6 billion and will reach $87.8 billion in 2016, capturing the biggest share, along with online computer and electronic sales with $87.9 billion. Overall sales are estimated to total $384.9 billion.

“Smartphone and tablet shoppers will be strong drivers,” the study said.

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This year, about 15 percent of US retail ecommerce sales are expected to be carried out through mobile devices, up from 11 percent in 2012, and by 2017 to surge to 25 percent, eMarketer estimates.

Despite the robust outlook, the research company estimated that only 6 percent of U.S. retail sales came from e-commerce in 2012, although a number of research sources believe e-commerce could eventually achieve a 20 percent share.

For this to happen, eMarketer said, “large spending categories need to gain more traction online.”

The report data show that large retail categories like auto and auto parts, and food and beverage “have disproportionately low sales online, indicating a lot of room for e-commerce growth.”

In 2012, online U.S. retail sales of auto and auto parts totalled $23.7 billion, food and beverage only $5 billion, and health and personal care, $10.9 billion. They are expected to expand by 2016 to $33.5 billion, $9.4 billion, and $18.8 billion, respectively, it said.

“With nearly three-quarters of U.S. Internet users already making purchases online, ecommerce growth will rely more on increased spending from existing buyers than on new spending from first-time online buyers,” the report said.