The research found UK online sales were worth £12.8bn last year, compared to just £9.24bn in Germany, the next largest market. France (£5bn), Italy (£780m) and Spain (£710m) lag the top two.
The UK also showed the fastest growth, soaring 75% between 2005 and 2007. However, France matched the UK growth rate.
"The UK has far and away the most developed online market in Europe," said Richard Perks, director of retail research at Mintel. "The main reason is the sophisticated online offering of the UK's food retailers and the fact that so many Britons now do their weekly shop online. The consumer boom of the last few years has also undoubtedly played a key role, when British shoppers were always keen to spend their cash."
Perks predicted in future Spain will see the largest growth, from just £710m last year to £3bn by 2012. France will jump three-fold from £5bn to £15,2bn over the next five years, and the UK will grow 200% to hit £40bn by 2012.
Although it is the largest European country, Germany will lag the leaders but still grow 69%, Perks said. This was because high street retailers had been slow to take up the internet, and credit cards are relatively unpopular, he said.
Mail order businesses have been the big winners, Perks said. "Amazon is the only pureplay internet retailer in the top 10 retailers, but five of the top 10 are mail order groups that have developed their e-commerce operations to stem the decline of traditional big book mail order and reduce overheads," he said.
Amazon is the third-largest online retailer, with Germany's Otto and Arcandor first and second. Tesco is the largest online food retailer in Europe, and more than twice the size of the next-largest, Carrefour's ooshop.com run by Carrefour. Mintel excluded eBay as it is a conduit from one consumer to another.