Police carrying out covert testing of security around London's Olympic Park have managed to smuggle a "bomb" on to the site on at least one occasion.
Dummy runs by London 2012 security staff achieved a 90 per cent success rate in foiling attempts to smuggle devices on to the site in Stratford, where the Olympic stadium, velodrome and pool are based -- meaning at least one attempt succeeded.
The disclosure underlines growing concerns over security at the Games. Such is the level of threat that all of the British army's explosives- search dogs are likely to be recalled from duty in Afghanistan to boost the security operation.
One security official said: "The issue of explosive search dogs is crucial -- they are the tools that will find the bombs, but we don't have enough of them. We have around 30 and they can only work for 30 to 40 minutes before they get distracted or bored. From what I understand, there will be virtually none left in Afghanistan -- they will all be here."
At the same time, troops will be used to search members of the public entering the Olympic site because not enough people have been recruited to help private security operators and the police.
As many as 7,500 military personnel will assist 16,200 security guards inside the Olympic Park, with 12,000 police and 6,000 more armed forces personnel on the streets outside. However, there are fears that the security operation will be overwhelmed by the logistics of trying to protect tens of thousands of athletes, spectators and VIPs over the six weeks of the Olympics and Paralympics.
Some security experts believe the greatest risk will come at sites where there is a lower level of protection, such as transport links and shopping centres.
Such an attack would also force the authorities to increase security at the Olympic Park to "level two". A security official said: "The real fear is that a bomb somewhere in London a week or two before the games will send security levels through the roof. Everyone will have to undergo a full body search, no vehicles will be allowed inside the Olympic Park and it will take hours to get inside."
The official disclosed concern over communications between the police, the armed forces and G4S, the Games's private security company, over what would happen in the event of a major incident.
Lord Carlile, the former independent reviewer of terrorism legislation, warned that terrorists could be tempted to target locations away from the Olympics.
"My concern is that we don't displace so many people to Olympic venues that it leaves a risk elsewhere. Proper policing levels need to be maintained in other areas," he said.
He welcomed the prospect of uniformed servicemen and women searching visitors. "It makes sense to use soldiers trained in carrying out searches on civilians, in places like Northern Ireland and Afghanistan," he said.
A Scotland Yard spokesman said there were "very clear roles and responsibilities" between the Met Police and the Games organiser, while a Home Office spokesman described its security procedures as "robust".
"We want to reassure everyone that we will leave nothing to chance in our aim to deliver a Games that the whole world will enjoy," he said.
A London 2012 Organising Committee spokesman said security training and planning were continuing. "The precise role of members of the armed forces is still being finalised," he said. "Locog will be conducting tests on the Olympic Park in the run-up to the Games to meet all health and safety requirements."