Staff members - who had received two days of intensive training operating X-ray machines to detect lethal weapons and explosives - failed to spot decommissioned hand-grenades and firearms during a test.
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Security officers asked to pat down a volunteer with a deactivated 9mm pistol in his sock failed to find the weapon.
Another guard picked up a fake improvised explosive device (IED) put through an X-ray scanner as a test and waved it around, despite being trained never to touch suspected bombs.
A trainee, who failed to keep track of which personal items belonged to whom, enabled a volunteer to put a mock bomb through the X-ray machine then disappear into the crowd.
All these members of staff were later passed as having successfully completed training. They were told to collect their security badge and uniform and were cleared to work in the Olympic park.
The whistleblower, who contacted Sky News, has extensive military and professional security training and is an expert in weapons and IED detection.
He said: "I can see so many security loopholes for this event. Security staff are given a very short time to achieve their training and there is a very slack approach.
“During my employment I planted pretend IEDs, decommissioned weapons, knives and other large metallic objects on students and sent them through the metal detectors.
"They're not being seen by X-ray staff and they’re not being picked up during physical searches, so the training is completely insufficient.
"The people making these mistakes are then given a tick in the box at the end of the day, sent round the corner to collect their uniform and sent home to await an email with their start date.
"Ninety-nine per cent of personnel coming through have no security background; many have language problems so they won't be able to communicate with ticketholders and many completely lack confidence.
"We found out we'd won the bid for the Olympics in plenty of time. If they had recruited and vetted personnel from day one I don't think there would be a problem.