London could see a fall in tourism in the years around the Olympics
The 2012 Olympic Games may have a "detrimental" impact on tourism in London, according to a report.
Cities that stage the games suffer a drop in tourism in the years around the event, the European Tour Operators Association (ETOA) claims.
If London followed the same pattern as Beijing, it could see more than 2.5 million fewer visitors and a loss of £1.5bn in revenue, the study said.
Visit London said: "It's impossible to compare Olympic cities directly."
The ETOA report, which said benefits of 2012 Games were "wholly illusory", looked at tourism figures for the past six Olympics, including Athens in 2004 and Sydney in 2000.
Whilst some of the events saw a peak in demand during the games, all saw a major disruption to their normal tourism market and none showed any obvious signs of tourism growth.
Beijing, the last city to host the Olympics, showed international visitor arrivals plummeted by 30% in the month before the games, compared with the previous year.
In the months after the games, the tourism slump continued with international arrivals down by more than 20%.
Beijing fared considerably worse than the rest of China in 2008, which was not a strong year in general for tourism in the Asia-Pacific region.
Following the Sydney 2000 Olympics, the city's tourism lost "significant ground" to other Australian and New Zealand cities, it added.
"We have yet to have a games where tourism has not been disrupted, and disrupted in a way that causes real harm," said ETOA executive director Tom Jenkins.
"Even in the case of Athens, where they carefully restricted new capacity, there were considerable losses before and after the games both in the capital and throughout Greece," he added.
Sally Chatterjee, Visit London's chief executive, said: "London is the world's most visited destination by foreign travellers and one of the most accessible cities in the world.
"We believe the London 2012 Olympic Games will bring an influx of new visitors to London."
The agency added that visitors will not feel "ripped off" during the Games as a Fair Pricing and Practice Charter will be in place.