Olympic terror fears have led the U.K. Home Office to carry out 500,000 background checks, but so far only 100 applications for accreditation to perform or work during the Games have been turned down, according to an article from UK news source Daily Mail.

Most rejections were made over concerns about applicants’ criminal records, but some were denied accreditation on the advice of the Security Services over concerns they posed a risk to national security, the article says.

According to the Guardian, the “threshold” for refusing accreditation has become more rigorous in the lead up to the Games, which start on July 27.

“This rigorous process has been designed to ensure those working at the Games are fit to do so,” said a Home Office spokesperson.“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.”

The security vetting process, which began in October last year, is now two-thirds complete and is believed to be the biggest operation of its kind since the World War II, the article says.

MI5 has been inspecting potential Games workers including about 10,000 security guards as well as the 10,5000 competing athletes, coaches and other officials from more than 200 competing nations, Daily Mail reports.

The 70,000 volunteers recruited by the London Organising Committee of the Olympic and Paralympic Games (LOCOG) have also been screened, the article says.

But while the main Olympic Park will be protected by the biggest peacetime security operation in British history, the Daily Telegraph has reported that senior security officials have become concerned that other less well-protected sites around the country could also become targets.

Meanwhile the Ministry of Defence has confirmed that a ring of ground-to-air missile launchers could be deployed around London to protect Olympic venues, the article says.