UK online retail sales are outstripping those seen in the US by percentage of all retail sales, according to analyst Forrester, and the UK is making up the lion's share of the total European online sales being generated.

Forrester says European online retail sales will reach 233.9 billion (£191.7 million) by 2018, up from 134.9 billion in 2013. This equates to a 12 percent compound annual growth rate (CAGR) over the next five years.

Whilst clothing will continue to be the largest online sales category in Europe, online groceries will be the fastest growing, surpassing consumer electronics to become the second largest category by 2018, Forrester said.

Of the 17 countries included in the forecast, the UK, France and Germany will continue to have the highest proportion of total retail sales coming from online sales. The predicted online sales growth across Europe, said Forrester, is driven by a combination of the increasing number of consumers that buy online, and retailers enabling improved e-commerce experiences.

UK online retail sales will reach 71 billion by 2018, up from 46 billion in 2013 and 51 billion in 2014, and representing 15 percent of total retail sales, (growing by a 9.1 percent CAGR from 2013 to 2018). By 2018, both France and Germany will see 10 percent of their total retail sales being made up from online sales.

By comparison, in a report published last month, Forrester forecast that total US online retail sales would reach $414 billion by 2018 - only 11 percent of all retail sales.

"Western European online retail sales growth will continue to outperform overall retail sales through to 2018, and will increasingly come from sales completed over mobile devices like smartphones and tablets," said Forrester analyst Michelle Beeson.

Forrester said the percentage of European internet users who buy online will grow from 65 percent in 2013 to 75 percent in 2018.

Southern European markets, such as Italy and Spain, will see "strong growth" in online retail sales through to 2018, said the analyst house, spurred by the growth in the number of online buyers and the emergence of "super-shoppers" in these markets - consumers who research and buy across multiple devices and digital touchpoints on a regular basis.