the coronavirus pandemic fuelled a boom in e-commerce, a United Nations study released on Monday showed.

Online sales accounted for 19% of overall retail sales in 2020, up from 16% a year earlier, according to estimates from the UN Conference on Trade and Development (UNCTAD) based on national statistical offices in major economies.

South Korea reported the highest share at 25.9%, up from 20.8% the year before. China had a 24.9% share, Britain 23.3% and the United States 14.0%.

Global e-commerce sales rose 4% to $26.7 trillion in 2019, according to the latest estimates available, UNCTAD said. This included business-to-business (B2B) and business-to-consumer (B2C) sales, and was equivalent to 30% of global economic output that year.

The pandemic led to mixed fortunes for leading B2C e-commerce companies in 2020, according to the report.

Data for the top 13 e-commerce firms, 11 of which are from China and the United States, showed a notable reversal of fortunes for platform companies offering services such as ride hailing and travel, which saw sharp declines in gross merchandise volume (GMV).

"For instance, Expedia fell from 5th place in 2019 rankings to 11th in 2020, Booking Holdings from 6th to 12th and Airbnb, which launched its initial public offering in 2020, from 11th to 13th," it said.

China's Alibaba remained atop the rankings by GMV, followed by Amazon in the United States.

Despite the drop at services companies, total GMV for the top 13 B2C e-commerce companies rose by 20.5% to $2.9 trillion in 2020, outpacing the 17.9% gain in 2019.