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The Olympic Games are at risk from athletes cheating under the direction of illegal betting syndicates, the director of Interpol has said.

Ron Noble said organisers should be concerned about gangs trying to fix results or parts of competitions.

But he said there was no specific intelligence terrorists or organised criminals would target the Games.

The warning came as police and military personnel carried out a number of security exercises on the River Thames.

Mr Noble, who runs the international police agency, told security officials in London: "I've got to believe since it's occurring so much in football and other sports we have reason to believe there is a risk of it occurring in the Olympics.

"The president of the International Olympic Committee (IOC) has said that he and the Olympics should be concerned about irregular or illegal betting occurring in the context of the Olympics."

He added that tighter border controls would be needed for the Olympics.

"Border checks are always a security concern when it comes to a country hosting a major international event, " he said.

"All major events such as Olympics require enhanced border checks."

But he said Britain did a good job of screening identification documents against Interpol's database of lost and stolen travel documents.

Some 44 officers from the Met's marine, firearms and air support units joined 94 Royal Marines in tests focusing on potential security threats ahead of London 2012.

The exercises saw rigid inflatables, landing craft, raiding craft and a patrol boat take to the water, as well as a Lynx helicopter in the skies above.

The ³ÉÈË¿ìÊÖ's Matt Prodger, who witnessed the manoeuvres, said: "What we're seeing here is a display of power, a display of intent, to send a signal out that London is ready for any kind of eventuality."

The officer in charge of Olympic security, Assistant Commissioner Chris Allison, said the river exercises were not in response to any specific threat.

But he added: "We would be failing in our duty to ignore it at Games time."

Typhoon jets, 13,500 military personnel and the Royal Navy's largest ship, HMS Ocean, will eventually be deployed during the Games.

"If we need the military support, it is there," he said.

"All of our planning is designed to mitigate against potential risks during the summer of 2012, and this is an example of where we will be using specialist military capability to support us.

"This exercise is not in response to any specific threat, but is part of our planning to pre-deploy certain specialist assets to bolster our operation."

The Olympic Games, from 27 July to 12 August, will represent the UK's largest ever peacetime security operation.

Crime and Security Minister James Brokenshire said: "This exercise forms part of the comprehensive testing and exercise programme that is crucial in securing the Games.

"We are leaving nothing to chance to ensure we deliver a safe and secure Games."

The Met estimates 9,000 officers will be on duty in the capital on peak days with 12,000 nationally on "Olympic duty".