Global business-to-consumer e-commerce sales will pass the 1 trillion euro ($1.25 trillion) mark by 2013, and the total number of Internet users will increase to approximately 3.5 billion from around 2.2 billion at the end of 2011, according to a new report by the Interactive Media in Retail Group (IMRG), a U.K. online retail trade organization.

The study estimates that business-to-consumer e-commerce sales in 2011 increased to 690 billion euros ($961 billion), an increase of close to 20% from a year earlier.

The United States remains the world’s single biggest e-commerce market, IMRG says, followed by the United Kingdom and Japan. IMRG estimates that growth rates in those countries will be approximately 10-15% a year.  But with China’s e-commerce sales growing more than 130% in 2011, it is only a matter of time before the Asian giant becomes the single largest market in the world.

In terms of regions, Europe is currently the largest e-commerce market in the world, according to a separate report released recently by the European Multi-channel and Online Trade Association, which said European online business-to-consumer sales posted 19% growth in 2011 to reach an estimated $307 billion, surpassing North America at $297 billion.

IMRG, meanwhile, is forecasting that France, Italy, Spain, Russia, Turkey and Poland will be the fastest-growing markets in Europe. It also anticipates substantial growth in Latin America, led by Brazil and Mexico, and the Middle East, led by Israel and the UAE.

Aad Weening, head of international at IMRG and author of the report, acknowledges that mature markets in Europe and North America face challenges from faster-growing economies elsewhere in the world, but says the growth of Internet usage and online shopping presents new opportunities in many countries.

“The future of e-retail is global and with the inevitable slowing of growth in several major markets in the likes of North America and Europe, it is important for businesses to understand where the future opportunities will be,” Weening says. “Worldwide, we are increasingly seeing trust and confidence in purchasing online growing and government and private initiatives brought in to support the development of the global digital economy.” He adds, “Online is now becoming an integral part of any country’s economy and should be considered so.”