The backgrounds of up to 500,000 people are being scrutinised in an unprecedented security screening designed to stop the Olympic Games being disrupted by criminals or terrorists, the Guardian has learned.

In what is understood to be the biggest vetting process since the second world war, the Home Office has so far refused about 100 applications for Games accreditation, mostly because of concerns about the extent of people's criminal records.

However, some people have been denied accreditation on the advice of MI5, which has to assess whether a person might pose a significant threat to national security.

The 500,000 figure includes anyone seeking employment at the Games, as well as athletes, coaches and officials from more than 200 competing nations.

The Guardian has been told the threshold for refusing accreditation has been set high, which means some of those working at the Olympics this summer will have "come to the notice of" the police or MI5 in the past.

"To be rejected, they have to pose a significant potential threat to the safety of the Games," said a source. "They won't be rejected on the basis that information is held about them.

"A judgment has to be made, not on the basis that there is an official record, but does this person pose a significant threat to security."

Police and MI5 have been taking a careful look at all those who may end up working at the Olympic sites. It is an obvious way for would-be terrorists to gain access to venues, and police are aware that terrorists may masquerade as casual workers looking for temporary jobs.

However, those involved in the security of the Games say they have found no evidence so far that al-Qaida sympathisers have tried to infiltrate the civilian workforce.

The vetting process began in earnest last October and officials are more than two-thirds of the way through the process, which is expected to be completed in the coming weeks.

It has been one of the core tasks of counter-terrorism officials but the scale of the operation, and the depth of the checks required, has made it a drawn-out affair.

Among those still to be vetted are many of the 10,000 security guards who will be employed by G4S, the private firm which is contributing 23,700 personnel at the Olympic venues.

A big recruitment drive was launched by G4S when the number of guards it was expected to provide grew from 2,000 to 10,000, after it emerged that the Games organisers, Locog, had seriously underestimated the number required. The 70,000 volunteers recruited by Locog, who are considered crucial to the success of the Games, are also being screened.

Home Office officials said that many of the 10,500 athletes taking part in the Games and those accompanying them were used to travelling to international events and were unlikely to pose any security problems.

There remain outstanding questions surrounding a handful of high-profile individuals, including members of the Syrian Olympic committee with close links to the Assad regime.

It is believed that discussions are continuing over whether to bar General Mofwaq Joumaa, the president of the Syrian national Olympic committee, from entering the UK.

Scotland Yard and MI5 are understood to have hundreds of investigations "live", with the Olympic security operation likely to reach a new pitch as teams arrive for training before the event.

It is understood that the security service has not set up a separate Olympic security unit, believing it would be wrong to draw a distinction between terrorism and Olympic terrorism.

The security service is said to be bracing for a possible deluge of intelligence from foreign police forces and intelligence agencies, who will not want to sit on any information just in case it reveals a potential threat to the Games. MI5 remains confident it will be able to cope, and the Home Office said it will leave nothing to chance when it comes to security.

"We are undertaking stringent checks on all those seeking accreditation," a Home Office spokesman said. "This rigorous process has been designed to ensure those working at the Games are fit to do so. We will leave nothing to chance in our aim to deliver a safe and secure Games that London, the UK and the whole world will enjoy."