2012 London Olympics
2012 London Olympics Reuters

A serious breach of security at London's 2012 Olympics site has been reported after undercover police officers were able to walk freely around the area with a fake bomb.

In a dummy run, the officers carried the device into the Olympic site in Stratford, east London, already in "lockdown" before the opening ceremony, according to the Mirror.

The "bomb", which contained no explosives, was designed to check security including electronic and human surveillance. The undercover team was able to move freely around the Olympic park area, where royalty, world leaders, athletes and fans are expected to gather in the summer.

The Met refused to comment on the breach.

The Olympic Delivery Authority said: "Testing is standard practice in all major security operations. Such tests have a key role in developing our capability to ensure that London 2012 is safe and secure and that we are best prepared to detect potential threats before and during the games. Members of the public with tickets should be reassured that such exercises are being staged to ensure their safety, our number one priority."

All army explosive search dogs are expected to be recalled from duty in Afghanistan. "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," a security official told the Telegraph.

The Cabinet Office will brief the Prime Minister, David Cameron, daily onsecurity at the games.

At least 7,500 military personnel will support 16,200 security guards inside the Olympic park, with 12,000 police and 6,000 more armed service personnel on the streets outside.

The Mirror reported that the cost of security has already gone up from £282 million to £553 million.