Over four million Britons are regular internet gamblers according to research from online measurement firm Nielsen/NetRatings.
Rules governing casinos in the UK are set to relax
The National Lottery website is the most popular, with 1.4 million visitors in an average month.
Well-known high street brands such as Ladbrokes and William Hill are also in the top ten sites.
A new government bill will require gambling websites in the UK to carry out age checks in future.
Gambling sites remain keen to attract new audiences and Nielsen's research found that those who upped their advertising activity online made it into the top ten sites visited, suggesting the ads were working.
TOP TEN GAMBLING SITES IN UK
The National Lottery
The Daily Draw
Time spent on gambling sites varied enormously depending on the type of gambling site.
While Ladbrokes and The National Lottery kept people's attention for an average of 35 and 17 minutes respectively, the average time spent on Casino Tropez was just one minute.
Nearly 70% of online gamblers are men the report found.
Online gambling is set to become even more popular thinks Gabrielle Prior, analyst for Nielsen/NetRatings.
"With more than 16% of all Britons online visiting a gambling site each month, it is evident that for some people visiting gambling sites has become part of their day to day surfing behaviour, and we could well see this trend increasing," she said.
With many of the websites based outside the UK it is going to be hard for the government to crack down on online gambling, she added.