One million more visitors are expected during the Games

Countries that stage the Games suffer a drop in tourism in the years around the event, the European Tour Operators Association (ETOA) study claims.

ETOA said regular tourists are deterred as they perceive a "full, disrupted, congested and over-priced" city.

The tourism organisation, Visit London, said the the report was full of muddled thinking and selective statistics.

Previously tourism experts predicted the Olympics would offer the UK tourist sector its "biggest-ever opportunity".

The Games present the chance to grow the visitor economy faster than would be otherwise possible Visit London spokeswoman

But ETOA executive director, Tom Jenkins, said while it was important the London Games were a success we must not "assume that there is any benefit flowing to tourism".

He said: "Olympic visitors tend not to be big consumers of sightseeing excursions; neither are they committed visitors to museums, historic monuments and other classic tourist attractions."

He added: "A reduction in the numbers of regular tourists halts the conveyor belt of satisfied customers bringing more visitors... word of mouth falls silent."

But Visit London's chief executive, James Bidwell, said: "The report is full of muddled thinking and selective statistics.

Adequate funding

"Our research in Sydney and Barcelona shows that the Games propelled their image as a destination in a way that no form of marketing promotion could ever do.

"It's true to say that the benefits don't come automatically, which is why we need to ensure adequate funding and proper planning if London is to reap the benefits."

And the Department for Culture, Media and Sport said it was confident that, with hard work, hosting the Games would "deliver a benefit of �2bn".

A spokeswoman added that an ambitious strategy would ensure a "sustainable legacy" for British tourism.