ETOA claims that, rather than enjoying a financial boost from the 2012 Olympics, the city's tourism trade could actually be damaged.
It has published research suggesting Olympic visitors tend not to be big spenders and their presence can deter other visitors before, during and after the Games.
ETOA executive director Tom Jenkins said: "Every city is unique, and each city handles the Olympics in its own way, but we have yet to have a Games where tourism has not been disrupted and disrupted in a way that causes real harm. 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."
But Visit London, which predicts £1.5bn boost as a result of the Olympics, has countered ETOA's claims.
Visit London chief executive Sally Chatterjee said: "It's impossible to compare Olympic cities directly. London is the world's most visited destination by foreign travellers and one of the most accessible cities in the world. We're expecting the London 2012 Olympic Games to deliver nearly £1.5bn in economic benefit to the city from increased tourism revenues over ten years."
- Visit London will honour venues, event agencies, hotels and visitor attractions at its annual awards ceremony tonight. The BT Visit London Awards 2009 take place at Westfield.
- The London Organising Committee of the Olympic Games (LOCOG) this week kicked off a £700m procurement process for suppliers in eight categories - facilities management and catering; production servies for ceremonies; security; services; sports; technology; transport and logistics; venues and venue logistics.