"The Olympics will increase the already "extreme" risk of a flu pandemic spreading in the UK, a report has said.
Britain has been ranked by analysts Maplecroft as second only to Singapore for the speed at which influenza could spread, because of its dense cities and status as a global travel hub.
The report said the "large influx of visitors" at Games time would raise the "already significant" risk of spread.
But the Health Protection Agency (HPA) disagreed, saying the risk was low.
"We have done our own review and we don't believe that there is a risk," Dr Brian McCloskey from the HPA told the BBC.
"We have sporting events and music festivals all around England and we had the swine flu pandemic at Glastonbury. We also looked at research from the Vancouver Games - neither produced any significant problems."
Dr McCloskey said that at Glastonbury in 2009, hundreds of thousands of people were densely packed in fields together for days at a time during the outbreak of influenza strain H1N1.
Whereas he said at London 2012 visitors would only be at the Olympic Park for four or five hours, so the risk was reduced.
Maplecroft's influenza pandemic risk index rates five countries at the "extreme" level of risk for the pandemic spread of the disease, with Singapore top, followed by the UK, South Korea, the Netherlands and Germany.
The study said that the Olympics would increase the danger of flu spreading because of the extra 5.3 million overseas tourists expected to visit Britain for the Games.
But it also found that Britain was in the top 10 of countries best placed to withstand any outbreak.
The Department of Health (DoH) said that the NHS had contingency plans in place for any eventuality.
"Outbreaks of infectious diseases during the Games have been very rare," a spokesperson said.
"The Health Protection Agency (HPA) responds to over 5,000 disease outbreaks each year and has robust systems and processes in place to do this.
"There is a comprehensive testing and exercising plan in place to make sure that all systems are ready."
Dr McCloskey said the chance of flu spreading during sporting events was quite low.
"It is much the same as it would be in any summer," he said.
"The fact lots of people are going to London during the Games, doesn't really change it.
"We have to be very careful about new diseases coming in but that doesn't mean we are high risk. Conditions won't be as such during the summer to make flu spread easily."
He added that the HPA would not be recommending any specific health precautions, other than those it already gives.
"Our normal advice would be if you feel unwell then stay at home. Then there is the standard advice we give across London at all times about sneezing, coughing and washing your hands.
"The simple advice would be, don't worry. But anyone feeling unwell - where they feel there is a cold, or have diarrhoea and vomiting then stay at home and don't bring it out with you.
"It's not an area people should worry about in any sense."
In January a series of reports, in The Lancet Infectious Disease journal, highlighted the risks of mass gatherings, such as London 2012.
Health experts said they can be a hotbed of diseases from across the world - the theory being that so many people, packed closely together, increases the risk of diseases spreading."