The Global Drug Survey found that 22.1% of British respondents had bought drugs online, of whom 44.1% had made their first purchase in 2012 or 2013.
The GDS sampled almost 80,000 adults, including 7,000 Britons, from 43 different countries, to help understand the trends in consumption of both legal and illegal recreational drugs.
Around one in five British respondents also said they had come into work feeling the effects of drugs.
The director of the GDS, Dr Adam Winstock, told ITV News people were using websites to buy drugs "for the same reason we go to Amazon".
I think it just reflects the increase in e-commerce that we see in every aspect of our lives. Drugs are a capitalist commodity, people look for best value but they also look for convenience, range of product, value for money and there is also the possibility of getting user ratings.
The next highest proportion of online buyers came from Denmark, with the UK well ahead of countries such as the USA, France and Germany.
Proportion of respondents who have bought drugs online
United Kingdom (22.1%)
Key global findings from this year's survey
Use of nitrous oxide - also known as Laughing Gas - at nightclubs has risen 20 per cent
One in four problem drinkers see their alcohol consumption as 'average' or below average
Hospital admissions related to using ecstasy in the last year almost tripled, while the drug's strength has roughly doubled
Only 10-20% of people trying e-cigarettes said they had quit smoking or significantly reduced their tobacco intake.
The poll also found a "huge level" of ignorance about government guidelines on safe drinking.
Dr Winstock said the survey showed that in the UK only "one in five drinkers pay any attention to those guidelines at all".
The UK was also among the country's where drinking had the biggest impact on people's working lives, with 46% of respondents saying they had gone into work hungover, second only to Ireland (50%).
The GDS found that one in four problem drinkers were unaware they were consuming too much alcohol, while 40 per cent of people do not know what their country's drinking guidelines are.
The variation among countries was stark, with German drinkers (65%) seen as particularly "clueless" about safe drinking, while their neighbours the Danes (8%) were the most aware of their government's advice on alcohol.