The credibility of the Olympic security operation being run by G4S is called into further question by claims that scores of trainees are being allowed to "cheat" their way through tests for the x-ray machines that detect homemade bombs and other weapons.
Recruits are being given only 20 minutes' practice on the real machines that will be used at the Olympic venues to stop visitors bringing in dangerous materials, or possibly an IED (improvised explosive device).
G4S insisted it has followed industry standards and that the tests were designed and approved by the Olympic organisers, Locog. It said it was not "uncommon or wrong" for trainees to repeat tests.
But the Guardian has been told that concern about the proper staffing of the x-ray machines was raised at the government's Cobra meeting on Monday, which involved senior ministers and Olympic Games officials.
With the Olympic opening ceremony just days away, police and armed forces have now been asked to "scope out" whether they can undertake more x-ray duties, and run the CCTV monitors too, if G4S cannot supply the numbers needed to operate the machines.
Gaps in the security operation have been emerging ever since G4S admitted almost a fortnight ago that it could not provide its full quota of 10,000 fully trained security guards. The firm hopes to provide 7,000.
The Ministry of Defence has mobilised an extra 3,500 personnel, and put another 1,200 on notice to deploy in 48 hours, but it is still unclear exactly where they will be needed and deployed.
On Monday Locog was left looking for 500 extra staff at the St James's Park stadium in Newcastle for an Olympic football match on Thursday. The organisers are likely to turn to the army and the police for help.
G4S was tasked with recruiting and training teams for the x-ray machines, which include the key job of "screener" – who has to look at the x-ray image of a bag and decide whether there is anything untoward inside.
Almost all the training undertaken by G4S recruits involves computer simulations, with only a small period set aside to work on the machines themselves.
Trainees have to pass seven tests, or modules, and in each module they are shown images of 25 suspect bags. They can only pass if they have a pass rate of 75% or more.
But because the test is a multiple choice with only three possible answers, if a trainee gets it wrong first time, it is much easier next time round.
"Only two people out of a group of 40 passed the exam first time," said one G4S trainee. "Those who failed were allowed to retake the exact same modules, endlessly repeating them until they scraped through. We were allowed to do the modules again and again, looking at the exact same bags, so by process of elimination you can pass the modules. You could retake the test as many times as you liked before the end of the day at 5pm."
The source said the trainees were all talking to each other during the exam to help them get the right answers.
"There were two invigilators but no attempt was made to stop people cheating. There was a general hum in the room with people talking to each other. By the end of the day, the majority of the people in the class, more than three quarters, had passed." This chimes with the experience of other trainees who have also contacted the Guardian.
The source, who has written a column for the Guardian about these experiences, said the training had not prepared recruits for working in these specialist roles.
"I don't think the training has properly prepared people for working as 'screeners' on the x-ray machines. It is a really difficult job, and it is also mundane, which makes it more difficult. It is extraordinary that you are only given 20 minutes on the actual machine before you are deployed. The real machines are very different from the computer simulations we used in training. Even when you have passed the modules it doesn't qualify you to work on the real machines."
G4S said the computer based modules used for the tests were "designed and approved by Locog stakeholders" and that the actual "operation of the machine itself is taught in line with industry standards".
Asked whether trainees were being allowed to cheat through their "virtual" x-ray training exams, a spokesman said the recruits could go through the test modules "at their own pace".
The spokesman said the trainers did not tell them the right answers, though they could offer "advice and guidance."
"It is not uncommon or wrong to repeat modules. Trainees are encouraged to ask questions and discuss concerns as this enhances their understanding of the tasks," said the spokesman.
"X-ray operators are given industry-standard time at the machines. All screeners spend three to four hours in simulation exercises."
However, the Guardian source added: "If you take the same test over and again, and the questions don't change, then it isn't really a test. During a subsequent session we were told that if we had cheated during the test we were putting lives at risk. But no attempt was made to stop this happening."
The president of the International Olympic Committee Jacques Rogge said on Monday he was in favour of the military being used to fill the gaps left by G4S. "They have found a solution. We are happy with it and we are very confident that security will be very, very good. The problem has been identified The company will compensate for the extra costs to the government and really it is time to move to a different issue. We are not going to enter in to a blame game. We are not going to point fingers because it is useless."